I have been flying off water a long time . Here are a few things I have learned about water and protecting your plane. Fabric coverings hold up better then films. Balsa-rite does help but just a thin coat of dope on the balsa helps. Make sure you leave air holes in the ribs of your wings and a place for moisture to vent if it does get into the wing. The biggest thing is to protect your receiver and battery pack from water. I use small plastic bags rubber band tightly around the wires to keep moisture out of the receiver and battery.
I always fly off of fresh water. if the very worst happens and your equipment gets wet just open it up and let it air dry.
If your engine gets wet take the glow plug out and flush the engine with glow fuel. I have never had a problem restarting an engine once this is done.
If a servo gets water in it take the servo out of the plane and remove the lower case and let air dry.
Over the years I have had more then a few flip overs and such on water. unless its a bad spill or crash just dump the water out of the plane open it up for a short time, let it dry a bit and go back flying.
The whole trick to trouble free water flying is having a reliable engine, "unless your using electric power".
I use four stroke engines and found they give a very reliable power source. I have used Saito and OS for the most part and found either brand work well.
If you are going to be flying a plane with floats it does take a bit more power to get off the water so up the engine size a bit. Overkill is not needed.
Never use wood prop. A wood prop hitting water is like it hitting concrete.
Make sure the wood glue you build the plane with is not water soluble. "Don't ask"
If you are buying a flying boat type ARF aircraft the very best way to check for unseen holes is to fill the plane with water. all holes show up this way. To seal the little holes I just use some GE clear silicone seal . stuff holds like iron and solves the problem.
And do yourself a big favor USE WATER RUDDERS.
Nothing like trying to bring a plane to you when the wind is pushing the plane away.
Have fun go fly!