Been working on this for a few days, made out of blue foam, plywood and fiberglass. Goes like snot and the electronics don't stay dry. Fortunately I have a few more ar6400's, I think I'll try some corrosionX on it and see if it'll last more than two minutes .

Micro Airboat (1 min 7 sec)

to be technical about it.....its not the water that is the problem.ive run electonics in pure water and they work fine. the problem is with the minerals and contaminents that short out the traces on the board. the ar6400 is hard to water proof. usualy what i do is use some cool melt hot glue and do the board that way. the ar6400 is hard cuz the servos. try petrolum jelly. might work.

Years ago(20 plus), my nephew and I were battling the same issues keeping the electronics dry and from corroding. We dipped the receiver pack and servos in ATF. After that we never had ANY issues with moisture or corrosion... And things kept working fine...FYI

I have used clear Krylon spray for years.
Remove the receiver from case hold by the antena and spray untill the paint runs off. let dry for 2-3 days.
You will have to clean the connections.
For a servo open and spray the board the same as the receiver. When dry reassemble using light grease under the output gear and RTV between the case sections and seal the wire leads.

Most of my 114 boats are done this way. I have not lost a receiver in over 35 years.

ATF?! that is funny! never thought of that.

spraying anything on an ar6400 would not be a good idea as the resistive strips for the servos' feedback will get coated and mess up the movement. it would be much better if you could tape up the hatch to keep the water out of the hull. only other option would be a separate receiver like a Deltang or one of the HK oranges. then you could coat it in waterproofing, or a small balloon etc.
it does look like it goes very well, must have a good power to weight ratio

My Home Made 12 inch Brushless RC Airboat With 3s 11.1V Lipo Verry Fast (7 min 58 sec)