The easiest way to get started is to go with one of the purpose built underwater cameras like a Sealife Reefmaster, Olympus TG-6, or Sea&Sea DG-6. Some of those have limited depth waterproofing and I think of them more as snorkeling cameras, although some can be upgraded with strobes and are quite capable. They will all take good pictures in the right conditions with decent light available, but the band of "good conditions" will be narrower and your options to upgrade will be more limited.
The next step up would be a compact or mirrorless camera in a housing. These will be much more capable depth-wise and will have more upgradeability to add strobes, wet mate lenses or lens ports for alternate lenses (wide angle, macro, etc) and that sort of thing. When looking at housing, there are a wide range of pricing options available, from Ikelite, Salted Line, and a few others at the bottom end to Nauticam, Subal, and Aquatica on the top end. We had an Ikelite on a Nikon DSLR for years and were mostly happy with it, but just downsized to a Sony RX100 VII in a Nauticam housing and I love it.
The top end is a full sized DSLR in a housing, and you can go nuts and spend $10k plus really easily with a full setup. This used to be the only option for doing the full range of photography underwater and having the most control over what you shoot, but the gap has narrowed dramatically with the recent introduction of more advanced mirrorless options and with camera makers now adding the top of the line autofocus technology to their compact offerings (like the Sony RX100 VII).
A big part of the reason we went back from a DSLR rig to a Compact based rig was that I found we were not really taking advantage of the full range of feature in the DSLR, and the gear necessary to have the right port for each lens, domes, the housing, strobes, etc. ended up being a huge PITA to travel with as baggage limitations keep getting smaller. With our DSLR, it took one large checked-bag size pelican case and one smaller carryon sized pelican case just to pack all the camera gear. Then you still have to carry dive gear, clothes, wetsuits, etc. Now with the compact based system, I can fit the camera, housing, two full sized strobes, and all my other gear for the camera in one carryon sized pelican case.
The real pros are Dubi and MrDubi on the outdoors board. I think she is shooting mirrorless now. My best advice is to 1) Be prepared for sticker shock if you want to buy everything new. This stuff is outrageously priced and you can spend thousands of dollars to get setup the way you want; 2) Go to places like ScubaBoard and Wetpixel to read about the pros and cons of various setups (
Underwater Photography Guide (uwphotographyguide.com) is a great resource to read camera reviews and setup advice); and 3) Don't be scared to buy used off of these forums, but also watch out for scammers. There are a lot of people constantly upgrading to new setups, so if you figure out what you want, aren't wanting to have the latest and greatest brand new thing, and can be patient, you may just find somebody selling a full rig of what you wanted for a steeply discounted price.
