liquid electrical tape, have a boat use it
I've put a lot of wire in boats over the years (who hasn't) and it goes like this: solder, flop on plenty of liquid electrical.tape, slide your heat shrink tubing over that and proceed to shrink. The excess liquid will ooze out and you will have an excellent waterproof connection. Takes a little extra time but well worth it. Amazing how little a exposure of copper in a marine environment brings on the green slime... I like Martini's idea of using the spray on panels, exposed areas, etc. Great stuff.