What is the best way to keep the switches and receiver + battery from getting wet during sailing? I am more worried about the switch as it is installed on the boat and will be the first item to catch on some water ?

thanks in advance

The best way I've found yet is to have a water tight hull and hatches. If you have dusty bilges the electronics will not be a problem. Some cover them with balloons and condoms. Electronics in use generate heat and will deteriorate when too much is generated. I leave mine in the open. One advantage over RC aircraft is that there is little if any vibration that has to be dampened with foam rubber.

Batteries don't seem to mind getting wet once in a while. Rx and switch should be mounted under the deck (velcro). Connections can be coated with silicon (Radio Shack, in a tube). On windy days, tape the hatch cover.

There's also a very handy DiElectirc Waterproofing material you can use, I've dipped my RX, Servos, and switches in it and they work fine. Batteries in a balloon stay pretty dry. The waterproofing stuff is AeroPlate. It's linked from the AMYA Suppliers listing pages.
Just follow the instructions and it works great.