I've not found any decent information on them, they seem to be a thing that racers cobble together, unless they they are known as something else perhaps. Here's a couple of pics where what they are and how they operate is clear enough. It's basically a deflector that's dipped into the water to provide drag. The rooster tail is a consequence which does look pretty spectacular:
Quite small things really.
The idea is straightforward. Fast airboats suffer from a fairly unique problem, in that at full chat the air under the boat can be enough to lift it, we all know that. But, so long as the engine is thrusting enough the bow is forced down, at high speed if you throttle off the wind wins and the boat can flip. So at the end of the course these racers deploy the brake just as they chop the engine, the drag keeps the bow down.
Now, for a model I've no idea how much use such a thing is in practical terms, but I'm guessing it could be useful for raising a smile