I think you would need to design a drag skeg to keep it on the water.
Even then, there would still be a great possibility that the model will catch air.
The few "cold power" rocket experiments we did were often scuttled by the high viscosity of water.
I did run a car for four years. Estes D engines...
There was actually a pretty heavy weight on the front wheel and suspension to
keep the nose down on the pavement. The ejection charge was channeled up and
over the engine to pop a drag chute back behind the vehicle after the run.
https://www.youtube.com/watch?v=00RwloRsIo4