Add the script to Simulation Flow (Ctrl/Cmd + F2) > FramesPost or StepsPost.
The script deletes all crest splash emitter ("RW_Crest_splash_01") particles below a RealWave surface's waterline. Particles, which flow over the surface's borders, cannot be detected and have to be removed with appropriate daemons:
emitter = scene.get_PB_Emitter("RW_Crest_Splash01")
particles = emitter.getParticles()
for particle in particles:
if (particle.isBelowRealWave()):
emitter.removeParticle(particle.getId())