PROPLESS, the reason the motor will rev to 6000 rpm out of the water and not in the water is cause the motor doesn't have any load on it. When the pump is in the water it puts a load on engine and won't rev as high or as quick. If you want more mph and rpm's you need more horse power from some engine upgrades and also some pump work like cutting impeller down from an "A" to maybe a "A/B" or a straight "B". I would start with some good perfomance exhaust, a cam and intake manifold, some mild head work and see what performance gains you see. Do a little at a time till you get what your looking for.
GOOD LUCK
DANO