RE: underwater cameras
The cheap waterproof ones if you mean the 35 mm, are unfortunately that....35 mm. I tried a couple in the past for when family visited, heres the big problem....they give you a cross hair target to look thru since you are never gonna get your mask up to that itty bitty viewfinder. I found that not even 25% of the pics even caught the subject in them. And that really sucks when you are paying to get film developed.
AS far as the digital, the amount of megapixels is pretty much what determins it. The higher end brands have features for color filters to suposedly remove some of the blue or green. Not sure how well that works.
AS far as video, without getting into high end, and a HUGE camera housing that looks like a piece of sewer pipe with plexiglass on the ends, a regular digital that also takes video works decent. There are 2 levels, the 720x360, and the 360x180. One is exactly half the resolution as the other, and makes a differnce in the video. To see the difference, on the list of videos on our website below, the "CHUNKY LOVE DIVES MIAMI" and "DIVING THE USS ORISKANY" were with the higher resolution CasioExilim, and check out the begining of "SUMMER OF LOVE" on the Oriskany, with the lower video resolution Intova. But both take about equal stills.
water darkness (depth)and clarity make a huge differnce. I have footage with the lower res that looks better than the higher res becuase of darkness. Low light underwater causes it to compensate by becoming EXTREMLY grainy.