IDK what system you prefer, I like Nikon. An entry level Nikon DSLR would run you 4-500 with a 18-55mm kit lens (which you might not want to use). You'd have to decide what lens you'd like to use, and you'd probably want a fast lens. If you like wide shots the Nikon 20mm F/1.8 would be a good choice, or the 14-24mm F/2.8 (this lens is $$$). A normal 35mm (crop sensor) or 50mm (full frame) would be a solid normal lens. If you want something longer a 85mm F/1.8 or the manual focus 105mm F/1.8 would work. I'd try to avoid zooms unless it's internal focus because of the lens port and the extra F stops will come in handy. If you're into marco photography obviously a macro lens would be a better choice.
What is going to kill you is the underwater housing. An Ikelite case without a lens port will be around 1500-1800 bucks. The lens port will be an additional 4-600. Don't think about one of those bags, it will flood and you'll be out a camera. if you get a DSLR get a housing.
If you're not going to shoot in RAW and edit in Lightroom, Optics Pro, or Capture One, a point and shoot might be a better option. You can get a Canon G7X with a underwater housing for around 800 and it's definitely capable of taking solid pictures. With point and shoots you just want to make sure the shutter lag is as short as possible, especially if you're taking shots of fish.
I wouldn't recommend buying this stuff used if you could avoid it. You're not going to save a whole lot on a DSLR unless someone is just dumping the camera and glass holds its value pretty well. You might be able to get a deal on a point and shoot but what are you really going to save? When it comes to stuff like this I think the warranty is definitely worth it. if something goes wrong it's really nice to know you can send it in to get it fixed.