Unplug your trailer lights before submerging the trailer. Nothing is waterproof. An underwater short is a bad thing.
Just more advice, place a set of truck keys on the boat key ring, and a set of boat keys on the truck ring. I've done this and NEVER needed the spare set, but I've seen friends have to call a locksmith out to them because their truck keys have wound up @ the bottom of the lake (and also, get to the lake ONLY to have to go back home, get the boat keys, then come back).