Last time I took the boat out 2 weeks ago it ran like a champ. Today
it wouldnt run over 1800 rpms. Talking about a 1990 merc 5.7 260hp -
alpha one drive. The boat starts and idles great. In throttle only it
revs as high as I dare but put it in gear and 1800 max.

Fuel flow. Check that the filter isn't clogged and that the pump is
delivering effective pressure. Fuel filters are cheap, if you haven't
replaced it 'recently' then now's as good a time as any. Consider getting
one that has a clear housing so you can see if there's gunk accumulating.

Doesn't take much fuel to crank up the revs when there's no load on the
engine. But put a load on (in gear, moving) and it takes a lot more fuel.
If the pump can't keep up or the flow's being restricted then you get stuck
at lower revs or it stumbles.