[L473ncy]

My mom uses some marine coating stuff on the electronics at her workplace (I think she called it "Conformal coating"). It's great for coating PCB's and making them waterproof to the elements you subject them to on the ocean (water, salt, T-storms, etc.). Not sure if it's readily available for consumers though but it should be available commercially.

I don't think this stuff will affect the heat dissipation characteristics too much if at all since marine autopilots should be capable of pretty high voltage/wattage/amperage (IIRC pulling up to 30A sometimes).