The trailer may well have been straight when he parked on the ramp, but after it rolled back & floated around a bit, its lucky they are still hitched together! . . and re the handbrake, well dealers should order a pallet of bricks and a roll of rope . . . tie one brick to one end of a couple of metres of rope, tiie the other end to door pillar with written instruction on how to use !
Didn`t think the ramp there was very steep. Must have left it idling while he was launching/loading the boat. I never trust the handbrake, always turn the motor off, in gear with the handbrake on when launching or loading the boat. That river gets a fair run in it at tide changes, been a few tourists drowned there.
If ramps have moss on them it doesn't matter if you have low range, great hand brake or wonder vehicle, it can slide into the water.
There was a ramp near Captain Cook Bridge in Sydney that was famous for launched vehicles - most were not insurance jobs but a lot were. (NOT in this case in my opinion)
Many years ago I watched a near new Ford fairlane go backward off the ramp at Wollongong somewhere, can't remember exactly where.
It went into the drink with the back wheels spinning and smoking all the way down, towed out by the boat that was not released from the trailer.
The boat became buoyant, lifted the has off the fairlane back wheels.
With no weight, there was no traction, into the drink it went..