You don't mention what plasma system you have.
If you have a system with a choice of different consumables...such as a Hypertherm Powermax....use the smallest nozzle size (FineCut) at near its maximum rated amperage, at the fastest possible cut speeds (your machine acceleration should be at its maximum capability as well). This will minimize warpage.
Your method with water will minimize warpage, but on most materials will cause a rougher cut edge and more dross formation.
Jim Colt Hypertherm