First Time Photographing Underwater

[serendipity]

Hello everyone.
I have plenty of experience photographing and own a Canon 7D. I am planning a family trip to Costa Rica, Belize and Panama in January, which will (of course) involve some snorkeling. I dislike swimming in general, and that's the main reason why I had never thought of underwater photography prior to this. I don't plan to dive either, just snorkel.
I would like to take photographs of the animals underwater for the first time in this trip, but am unsure as to which gear would be best in this situation. I am afraid that the "zip locks" sold for underwater photography will leak, so I am looking into renting or buying proper underwater equipment. I have looked into brands such as Aquatica and Ikelite, but still feel insecure. How safe are these? What are the chances they will leak and ruin my camera?
Also, I have come across several guides - including this page - for underwater photography, and have noticed some things are very different from what I have had experience with before - for example, should I should auto WB, or select a specific custom WB set temperature? My lenses are a 50mm f/2.8, 100-400mm f/4.5-5.6 and the kit lens, 18-135mm. Which one is better for underwater photography? Would f/2.8 be too small? I also read that one would need a wider angle for underwater, so my 100-400mm would be of little use, right? My last question is: for underwater casings as the ones I have mentioned, how does the lens O-ring work exactly? I am assuming I would need a specific size for a particular lens, correct?
Thank you so much for any help and sorry about all the questions!
I would really appreciate any answers.
- Amalia

[bvanant]

I would rent something simpler than a housing for your 7D. There are lots of places that can rent you a small P&S system for you to play with in the water and getting a large rig under control while you are "not liking" the water will be very painful.
BVA

[serendipity]

Thank you for the response!
I've considered a Point-and-shoot, but I'm concerned about image quality, so that's the primary reason why I wanted to use my camera...
However, I am willing to give a simpler compact camera a try underwater (despite being disappointed with results in a previous attempt). Are there any systems you would recommend specifically?
- Amalia

[bvanant]

Well this is a complex question. I think that for snorkeling only and assuming your breathholding capabilities are not awesome (you say you don't like swimming) it seems to me that worrying about image quality is not the place to start. In terms of rental, if you can get hold of a Canon s100/G12 in one of the Canon housings and a strobe you should be able to do pretty much as well as you could with a good DSLR given the 15 seconds or so that you will have to compose, and shoot.

Bill