I can't see why you would need it water tight? Are you going to submerge it in the lake?
I have had those connections when using extensions just lay on the ground (not in a couple inches of standing water) in the woods or in grass through all day rains and thunderstorms for weeks and months at a time, and have never so much as kicked a breaker. Have not electrified the entire property either. The same with extension cords in general.
If you need to worry about such things just jam it in a Walmart bag and slap a tie on it.