Acceleration Problem on 25 hp Mercury

[lightytoes]

I have a 2000 model 25E Mercury outboard. The problem is outside with water hooked to the motor it runs perfectly. When the motor is cranked in the water it will idle perfectly, when you give it full throttle it will not accelerate but to about 7 mph wide open. Put it in neutral with fast idle it will open up wide open, drop back in forward all it will not open up. Has new prop, no difference, still will not run over 7 mph. First two times out this year it ran perfect. Third time I backed into the water, This started nad has not ended yet, any ideas?