Try this. After starting the CL130 US Coastguard (with floats), press F2 ONCE (a quick tap). I find that it stops it from going forward (stops it if it IS moving forward too) and it won't go backwards either. Even after pressing F1 to return the throttles to the the STOP position, it doesn't move forward again.

I can't remember if this worked every time, but I do know it works.

Mark