Im an considering buying a 39' motoryacht with 2x450HP DD engines.
It's equipped with Waterjet engines.

I can see that most other motoryachts in the same class can perform
around 30 knots max speed and about 25 while cruising. But the yacht i
am considering only goes about 15 knots. The sales people says it's
because of the waterjet is slower then Propeller. But it looks more
like a 50% reduction then anything else. It's this normal?.

Also the yacht makes a lot of noise. How are other yachts?

I don't know much about motor yachts used to do sailing.