Sorry to have dropped the ball on your question.
I read it when you posted it, but got sidetracked before answering.
There's no keystroke (like parking brakes) to stop you. The brakes don't work on the water. But swishing the water rudders left and right will help slow you down a little.
Whenever I want to stop the forward motion of the plane, I zero the throttle, of course. Then I lean out the fuel mixture to just above cutoff, and feather the prop. If the wind is not blowing too hard on the plane, it should stop moving.