What happens is the dirt/sand/silt will get in the clutch case and if not cleaned out after the return trip home it will solidify and your clutch will not longer shift out or in for that matter.
Luckily it is not that big of a deal to prevent and a little more work to fix.
To prevent it just pull the drain plug after a return trip from anywhere you were in water and if necessary wash off with a hose and either leave the cover off until it dries or the plug off and make sure to replug before the next trip.
To fix it access the clutches and you will see the spring seat stuck in the movable sheave after removing the movable sheave and from the opposite side just take a 3/4" wood rod and tap each side gently until the stuck spring seat come out.
Then take some scotchbrite and some warm soapy water and clean the bushing in the movable sheave and clean the bearing surface of the spring seat.
If the surface of the spring seat is heavily corroded use some 320 emery cloth and polish it up.
