hey carl,
it might also help if you posted a bit about your budget... how many cameras are you hoping to get and what do you want to spend? also have you thought about strobes, and renting those out separately?
in my experience there are 3 types of divers when it comes to underwater photography:
1. people with their own gear... SLRs with strobes or point and shoots with strobes but pretty much self sufficient. not really your target market, but if you had a couple of extra strobes to rent out, you might capture a couple of people who dive with only one, and want to see what it's like diving with two before investing in a second.
2. people who have a point and shoot that they use on land. before their vacation they bought a housing for it thinking "wow this will be great! i'll get some underwater shots and they'll be awesome". little do they know with out a strobe, unless they are in 10m or less they probably wont get too many decent shots... this is where rental strobes would be great.
3. people who don't have anything and don't know anything about underwater photography. (your main target) for this reason i think your best bet is to stay with a point and shoot system... the canon G series would be great, but not knowing your budget it's hard to say.
you probably already have plans for this, since you're planning to rent cameras, but having someone available who can teach people a few tips on getting great shots underwater.. as well as basic stuff like how to actually use the camera, would be awesome.
i've been to quite a few places where they have like one rental camera. you pay your money and they just hand if over, and drop you in the water. most people have no idea what they are doing and their shots usually show it. that being said, most people are just so happy to have underwater pictures they don't really care what they look like.... until they see someone else's who knows what they are doing.
i guess what i'm trying to get at, is what are you hoping to get out of renting cameras.... are you just hoping to make a bit of extra cash by renting out some basic systems? or are you hoping to be come known as a place where people can learn, and get some great shots. i think being that you're in moalboal with lots of other competition, this could be a great way to set yourself apart.