From what i can tell a video resolution of 720p is enough, and 1080p is even better but not actually necessary in most cases. The next thing i was told affects quality is the framerate, and i think my gopro off memory does 30fps(frames per second) and that seems to be fine. So after those two have been taken into account i would just look for reviews on the camera and the brand to see what the quality is like in other peoples experiences. And also with the depth, id make sure its something decent like over 25 meters or so just to be sure you shouldnt flood it. and lastly, IMO getting a warranty isnt a bad idea. Hope this helps helps and im no expert but thats what ive found
Edit- this is all in relation to video stuff as that was my main priority when i was looking for a camera. Im sure some of the other guys will know more about what to look for when going for still shots