I'd say it's more important to just repack the bearings when your finished than rely on a type of grease to save the bearings. Make sure the seal on the inner bearing is on right, thats really the only defense so it's important. My local wheeling conditions are 2' of water and sugar sand, so this is something I've had to deal with quite a bit. <img src="/forums/images/graemlins/notooth.gif" alt="" /> I forget who makes it, but alot of the swamp buggy guys use this pink synthetic grease and swear by it.
I've tried about every grease out there (including that pink stuff)to prevent this and it's a losing battle. Just repack when your finished and don't worry about it.