Let's Talk About Gear
This post will be fun because I get to talk about photography gear, but it will also be sad because I have to talk about how this type of photography can get expensive very quickly.
If you are going to take photos underwater, you're going to have to take the equipment you would normally use into an environment that will ruin it instantly. Your camera and lights will all have to be sealed and waterproof, and you will need to have access to all the buttons and dials on that equipment.
The first piece of equipment you're going to need to take photos underwater is a housing. A housing for a camera is basically a waterproof box that your camera, or camera body goes in to keep it dry. Small cameras like a Go Pro have housings that contain the entire camera and lens. DSLR cameras have the housing which holds the camera body, and a lens port and port extension which covers the lens and keeps it dry. The port and port extension attach to each other and to the housing.
Housings can vary in terms of cost from a few hundred dollars to several thousand dollars. If you're shooting with a DSLR, you'll have the additional expense of ports, and port extensions to cover the lens you chose to shoot with. After you add lighting, sync cables, and other accessories, you will easily have several thousand dollars worth of equipment going underwater with you.
The least expensive option that I've found to take your camera underwater is a bag system. I don't know much about them, as I have not used this my self, but I have seen tutorials on You Tube, and have seen some photographers achieve some good results in a shallow pool. These systems can be purchased for less than $50, but The one pictured below cost about $429 on B and H photo.
Another option is to get a housing for a compact camera or even an iPhone. These housings start at around $100 and can cost as much as $600 or $700 or more. Remember, these are just the housing, and does not include strobes, but at least you don't have to worry about parts for lenses.
More expensive housings are typically more ergonomic, allow easier access to camera controls, will have better through the lens viewing capabilities, will allow you to make adjustments while looking through the lens, and other design factors to make it more like shooting above water. Additionally, higher quality housings often can pull a vacuum which essentially makes it almost impossible for the housing to flood and ruin your equipment. You pull the vacuum with a hand pump, and when you do this it basically sucks all the air out, and pulls the parts together good and tight, and holds them together. Many of these types of housings have an indicator light that turns green when there is a vacuum in the housing, and red when there's air leaking in. This vacuum system has saved me from having a flooded housing on more than one occasion.
The housing brand I use is Ikelite. They make high quality housings for compact cameras that cost around $700, as well as housings for DSLR cameras The housing for the Canon 5D mark 3 that I use costs about $1500. Once you add a strobe or two, a dome port for a wide angle lens, strobe arms, and sync cables, it will end up costing close to $4000, and that's not including the camera body and lens. So it's definitely an investment. Other brands like Subal, Nauticam, and Sea and See make housings that are over $3000 and some over $5000 for just the housing. No dome ports, no lighting, just the housing. Again, this is very expensive, and it's difficult for the average person to invest in all this gear.
Another thing to consider with DSLR housings is that they are very specific, and usually only fit the camera make and model it was designed for and nothing else. So once you have invested in a housing, you better like the camera that goes in it, because if you decide to change your camera body, the chances are it wont fit in the housing. If the manufacturer comes out with a new version of your camera and they move a button a couple of millimeters, change the size of the display on the back, or if ANYTHING on that camera body changes in size or location, it won't fit in the housing, and you'll need to buy another housing to fit that camera. Most camera manufacturers aren't as concerned about keeping the camera body identical, they just want to make a better product. If they have to move some things around to do so, then so be it. Most people aren't shooting underwater, so the amount of customers a change is design affects is minimal. The good news is that if you buy from the same housing manufacturer, all the port extensions, and ports, and cables should fit, so you'll just need a new housing.
Here comes the bad part... in my opinion, it is very difficult to get high quality underwater images without a DSLR (or mirrorless camera) in a housing with strobes. Cameras housed in bags, or Go Pros will just not give you the same results. For one thing, a DSLR has a better sensor. If you don't have strobes to light your images, the camera will need to increase the ISO, or sensitivity to light, and this will result in more digital noise. Also, the ability to change lenses depending on your needs has its advantages. For example, if the water is cloudy, you'll want to reduce the distance between you and your subject as much as possible to reduce the amount of cloudy water between you and your subject so that your image will be clear. Changing lenses is nor really an option with a compact camera or a Go Pro. I've seen people try these shoots with less expensive equipment, and their images have poor lighting, and are very grainy, hazy, and have a lot of digital noise. This is unfortunate because the expense of using quality equipment underwater prevents a lot of people from trying underwater photography. It's also an issue because who wants to invest that much money into a hobby that you may or may not continue. My first camera I took underwater was a disposable camera in a plastic box. At that point I was taking photos while scuba diving, primarily in the ocean. Then I got a point and shoot in a small housing. Then I added a strobe that was triggered by a fiber optic cable, and I could never get it to work and sync with the camera. Then I finally took a leap of faith and got an Ikelite housing for my Canon 5D. This set up had 2 strobes with a sync cable, extension arms for the strobes, and a dome port that could accommodate a wide angle lens. Fortunately, I've been able to get years of use out of this housing both in the ocean photographing sharks or smaller animals, as well as working in the pool with models.
Did you know that wavelengths of light get filtered out by water the deeper you go? This is particularly a problem when you are scuba diving. By 15 feet red wavelengths are gone, and by the time you get down to 90 feet or so, all that's left are blue/green wavelengths, so everything looks blue down there. This isn't too big of a deal if you're shooting in a pool, as far a the color goes, but if you're going to want to light your subject like when you shoot in a studio, you're gonna need some strobes, and they're gonna need to be waterproof. The amount you spend on strobes will depend on the size of the strobe, it's power and the amount of light it puts out, and its recycle time, which is how long it takes to recharge and be ready to fire again after firing. You can budget at least $600 a strobe, and more if you're using a DSLR.
In addition to the strobes, you will need a way to trigger them. Sync cords attach to your housing, and connect to your strobes. A connector inside the housing fits into the hot shoe of your camera like a speed light would, and the signal to fire is sent from the camera to the strobes via the sync cable. You can also purchase optical triggers that can fire off camera strobes. To get a little more creative, you can attach them to a really long sync cord, and set up lights all over the place.
I strongly recommend using strobes designed to be used underwater when shooting in a pool. I have seen lighting setups where normal studio lights were used, and were positioned above the water. In my opinion this is potentially very dangerous. If those lights aren't super secure, and if they fall into the water while still plugged into an outlet, the results could be deadly. To use this type of lighting, you would need several people whose only job was to make sure the lights didn't fall, and to pay attention to the power cords.
An underwater photography rig can get pretty tricked out if you want to add all the bells and whistles. In addition to the camera housing, lens port, and strobes, you can add things like floats to make the rig neutrally buoyant underwater, snoots to reduce the amount of light coming out of your strobes, diopters to magnify your subject, off camera strobes, and on and on. For underwater fashion, I recommend a wide angle lens like a 17-40mm or a fisheye lens, at least one strobe, and maybe a Go Pro to get video, and behind the scenes images or video.
Whatever version of an underwater housing you end up with, be sure to get insurance on your equipment. This gear can be very unforgiving if you aren't careful. For example, if you don't assemble the housing and lens ports just right, it can leak and your gear is ruined. A small fiber, a hair, some grains of sand, any of that can get on the o rings and keep the parts from sealing the housing properly. This happened to me once, and when your down 60 feet underwater on a night dive and your camera suddenly stops working, it's such a deflating, soul crushing feeling.
One other thing to consider is the amount of assembly required for your housing. Bag systems and compact camera housings are assembled fairly easily. Pop the camera in, click the handle down, and you're ready to go. Large DSLR housings like the ones I use, are more difficult to assemble. The first time you do it will require a lot of patience, and will likely involve a significant amount of profanity. There's lots of screws, and caps, and o rings, and lubricants, and gears, and nothing seems to line up properly. Sounds like fun, right. The first time I assembled my housing, it took a few hours. Now I can have it set up in less than 10 minutes, so it gets a lot easier with time. I'd recommend, for the first few times, just putting the housing together with no camera in it. Then you can test it in a bathtub or pool to make sure you've got everything assembled the right way so it doesn't leak. Then you can go back and try it with a camera in it. Just be sure everything is working properly before you get in the water with your rig. The hot shoe connector needs to be in place, the zoom gears need to be working, and don't forget your memory card and battery. Yeah, I've done that.
There's a lot to consider when choosing underwater photography gear. Cost. Quality. Size. It all figures in. Oh, and don't forget about weight, because this stuff can get heavy, and if you're traveling with it, be prepared to pay overweight fees at the airport. I hope I've been able to answer some questions, and maybe you have more questions after reading this. Let me know if I can help with any underwater photography gear questions you have, I'm happy to help.