comming from RC boats I have a few things I have to say about things being said... Stick batteries could care less about being live and submerged in FRESH water.. same with elec motors .. hell most of us break in new elec motors by submerging them hooked up to a batt pack and leaving them overnight.
now salt water is a whole different animal.
as a comparison.. I have an RC submarine that is just over 3 years old.. the elec motor, stick battery and 2 servo's are OUTSIDE the sealed inner chamber ... when this sub is in the water, the motor, servo's and battery are submerged in water the entire time. when I first started running it I was really anal about spraying out the motor, servos and checking the battery pack. for the last 2 years I've been waiting to see when these parts would die.. so all I do is leave the hull open to dry after a day at the lake , and make sure the batt pack is dry before charging.
your mech boxes.. there should be enough grease in them to prevent rust... and if the mech box is sealed enough to keep the water in... how did it get in? (aka it should run out)
the only thing I honestly could see causeing damage would be water getting behind a BB and hydrolocking as the BB tries to fire putting possibly too much strain on the spring and gears.