Nitro,

Using a 6000kv motor with a 6v battery doesn't result in extremely high RPM. Because of the rather low voltage of a 2S lifepo pack I need that high kv to get the RPM output that I need. So 6000 x 6 = 36 000 Rpm.
The same story in a car on 6 cells would give you 6000kv X 7,2v = 42 000+ rpm's. Most boats are direct drive without some sort of gearbox that cars use ( diffs and pinions and spurs ) so there is a difference here.
Higher RPM leads to cavitation indeed , but that mostly is because the prop that is used is to small or the strut is in a bad position or the weight distribution isn't correct. RTR boats like the SV and stuff don't have a correct weight distribution most of the time.