honestly it's more about the lighting and how fast you can set your shutter speed to. just make sure you get a camera with a full manual mode so that you can manually set your shutter speed and aperture settings.
Then it's a matter of lights!! really really bright lights. the brighter the lights, the faster your shutter speed, means the sharper your image. IE: you "freeze" the fish with a really fast shutter speed so you don't get what i see all the time here which is referred to as 'motion blur'. I use a tripod mounted strobe but then again i am a photographer. it's a give and take.
long story short (or maybe not!! LOL) it's not about the camera. so don't spend tons of money on a camera thinking photos will be better. in fact the camera is a VERY small part of the equation.