Dave,
Yes it will move forward at idle in the water unless you have a head wind.
Water has a lot less friction than the land does

I allways put my receiver in a plastic bag with a rubber band over the servo wires to keep out water, you may want to think about you battery switch also? will it get wet?, If it's mounted on the fuse side? the anwser is yes. you may want relocate it into the fuse w/a wire to turn on and off

lastly is the wing saddle, High wing plane are notorius for letting in water at the wing saddle, you'll want to make a gasket of some kind to slow water for getting inti the fuse,

Silicon work great for this,
1st Tape wax paper to the wing @ saddle area
2nd Put the silicon on the wing saddle,
3rd Bolt or rubber band the wing on
4th Let the plane is over night or longer 24 hours is best!
5th Un-tape the wax paper form the wing remove wing/wax paper carefully and trim the extra silicon the squezzed out, Now you have a nice gasket
6th Go fly have fun

Good luck