2000 Air Nautique engine issue

It sounds like the boat is going into
limp home mode (the computer senses something
wrong and kicks it down to idle so you
can get home but don't blow up an engine).

I don't know what is causing your issue.
In my case, I was two quarts low on oil.
When I was at idle or low speeds, no problem.
When I would speed up, apparently the low oil
was detected.

Perhaps not your issue, but real easy to check
you fluid levels. Other common cause is
overheating, but I would expect that at
different speeds, not just high rpms.

BKH

Yes, but a faulty limp mode sensor can cause the problem. Something a tune-up or fluid check will not solve. Disconnect the wire from one sensor at a time to see if you problem goes away. If so, replace that sensor (really just a switch to ground).

The temp switch looks like the water temperature sender and is located in the intake manifold water jacket. The oil pressure switch is "tee"ed off the pipe leading to the oil pressure sending unit.