I really don't think that JK park brakes can be relied on to be the same from one identical vehicle to the next or even one vehicle from one week to the next.
I launched my boat with mine and didn't even give this a thought. But then, mine's an auto.
Why don't you whack the boat on and zip down to the steepest boat ramp or slope you know?
Take a passenger and some wheel chocs if you are concerned about becoming another U tube hit.
Note that using the engine compression as as a park brake can be risky even though it is always a good idea to put it in low gear in these situations.
And if you are still concerned, try parking on the ramp in 1st gear, low range with the park brake "wrenched" on firmly
Personally, I reckon that would satisfy me. Pretty hard to roll then even if the engine was a bit stuffed and leaked compression.
