Your Nikon D3500 is a pretty decent camera. So I am thinking that you are looking at something that takes good stills.
As stated by others, underwater photography is not for the neophyte. Good buoyancy control is critical. For good shots, you need to be in the right spot.
Also you can get task loaded. That is your brain gets absorbed in photography and forgets things like breathing, deco, depth, where your buddy is and on and on. You need to be pretty dialed in on diving.
Also underwater photography is challenging. Long range tends to be 5’. So you have to get close to things. Also you lose color as you go down. At about 70’, everything is green or blue. If you want “natural collars”, you need a light source. That means a powerful video light for video or a strobe for stills or better off 2 for better coverage. And then there is stuff in the water causing backscatter.
And there is expense. Underwater photography is expensive.
To get more education, you can go to the Backscatter web site and they have videos, reviews and a wide range of products so you can get an idea of what is out there.