Mostly, the prime numbers are to reduce parasitic vibrations. It's done in jet turbines, but it's not so critical in water jets.
The "shore hardness" figure shouldn't be too high (brittle), or too low (too flexible)
Easy cast urethane resin is about right for an impeller. It also sands really well.
http://www.topmark.co.nz/articlelive...Full-View.html
I see they've added a clear easycast, that would be great for moulding jet units with !