Not sure on the exact RPM but your main problem here is scale.
Lets say a 1/48 scale.
A full sized sub would be traveling 10 MPH a scale sub would be traveling 10/48 or .2 MPH which would look like it is barely moving.
So we tend to run (Not me of course) our boats faster to not be bored out of our goard!!
The props have to spin faster to do this. I think water density may have something to do with it as well.
But I could be totally full of crapola on this.