I think that LiPos are not water proof and I have heard that if the LiPo itself (not the casing, but the polymer) gets wet bad things happen.

Perhaps coating the battery in plastidip and inspecting it before wet weather usage would be adequate?

Although, now that I think about it sense the E-Revo has enclosed batery trays you might just be able to seal them (silicone caulk/RTV along all seams, make sure there are no gaps)