Throw in the tide as another variable involved in launching....

In other words, this is probably inevitable that the trailer will get
wet no matter how careful I move the trailer into position. I see...

the wiring harness.... If you use an reverse lockout solenoid, I
recommend that you use the standard 4-wire trailer set-up and add a
2-wire plug (available at any auto/trailer/etc. store) for hot and
ground to the solenoid. Using two plugs, you can unplug the trailer
lights, but keep the brakes attached... if the solenoid goes under
water you probably have wet carpet in the tow vehicle...!!

You have a good point. This means I cannot simply disconnect the
wiring between the towing vehicle and the trailer; I need to at least
keep the connection with the trailer brake. The fact is that my towing
vehicle is a compact SUV (1996 PathFinder) instead of a full-size SUV,
and the fact that the 18-ft boat is relatively big for the SUV. This
means I should worry about the towing vehicle being dragged down into
the water by the weight of the boat/trailer. Now, I just have to fully
understand your wiring instruction. Honestly, I don't quite understand
the details of your instruction. I probably need to print your
instruction, and bring the instruction to an auto store and ask around.

Thanks for the info.

Jay Chan