I'd like to help you out, but this is sort of unfamiliar territory for me, and probably most of the other guys on this forum I'd suggest you take a look at Tamiya's TLT-1 axles. Or at least consider them. I'm not sure how they'd scale out for a 1/5 scale model like this, but they are pretty common and have plastic housings. I wouldn't worry about running them in water. Just run the cheap bushings so you won't have bearings rusting up. The gears will be fine with the water, but you might want to tear them down once in a while for maintenance, like greasing. Here's a link to the axles. You'll have to decide if they might work for you.
http://www3.towerhobbies.com/cgi-bin...?&I=LXVBT9&P=7 Not sure if you want solid axles, or if this model is to have independant suspension? If you intended to run the plastic wheels and tires that came with the model, you'll need an adapter hub of some kind to drive the wheels off the axle. Not a huge deal.
Here is a pretty cool build you might enjoy. It's not much what you are after, but maybe you can find something of interest in it.
http://www.scale4x4rc.org/forums/showthread.php?t=52119 It's on a different forum, that might well be worth your time to check out and ask the same questions you asked here. The crowd is a little more diverse and possibly someone might have good advice.