I hope this works.
Boat Launching for Dummies
They told me not to get the trailer too deep into the water.
I wish I had pictures of a similar incident at Homestead FL in the mid 60's
A guy came with a brand new boat, trailer and pickup and launched the boat by hitting on the brakes and letting the Boat slid off the trailer, however he did not have a rope attached and the boat went out into the the launch area. When he jumped out to try and catch the boat, the pickup and trailer rolled down the ramp and flooded the truck before he could stop it. The boat while he was dealing with the truck filled with water because he had not put the plug in the boat .