I work at a pro shop with a launch and after seeing this issue many many times Ill add this piece of advice: If you have a big boat, or a little truck, or the trailer wheels maybe get stuck on the start of the concrete ramp, the 350 horsepower engine on your boat can really help your tow vehicle out. Just give the boat some power to help push the thing outta the water.
Just in case this isn't obvious to everyone: 1. Make sure to shut the engine off and prop off before leaving the water. Honestly, as soon as the truck starts moving would be okay to shut it down. This tip is just to get it moving. 2. Not much power is needed...at all! This is really just meant to make the boat feel lighter on the trailer and the truck should still be the main tool. just a little bit helps more than you would think
Most common use for this application occurs when a smaller vehicle backs in deep enough to float the boat on, wheels get wet, and then you start slipping.