I got this electrical waterproofing spray at work (electra-coat by lubemaster), the liquid sets up into a scotch tape like plastic film, works great for waterproofing battery terminals, boards, and the like.
I'm considering spraying my boards with this stuff, then cutting it away between the optical contacts.
We had a hoist control remote fall into a hot rinse tank at work, took it apart, blew it out, worked. Sprayed the inside with that plastic film, they got it wet twice since and it hasn't stopped working.
I'd say it's probably the best bet for waterproofing PTWs