Tranny slipping or something else?

Hi guys. Just bought my first inboard, an 81 CC Southwind. We had been running it fine for several days, and were out yesterday when we "lost" forward gear. Reverse works fine, but put it in forward and it won't go any faster than it would at idle. Tried to rev it and the motor revs but prop must not be spinning? We were able to idle back with what little forward gear we had and trailered it. Fluid was fine. I know you're not supposed to do this, but we ran it BRIEFLY (with a flusher) to see if the prop was spinning like it should at higher RPM's, and it spun fine (out of the water). Any ideas? I can maybe describe it better if I need to. Thanks in advance for any help.

I guess your saying it is engaging in forward...but under load , not not spinning faster that forward idle...right? You are getting some forward thrust though?

If it's more than something simple...others on the site will be of more help...hang in there and you will get responses!
As far as simple trouble shooters...check linkage to see that forward clutch FULLY engauges...and PROPERLY check level!
The level should be checked when boat is in water and after warmed up(cold readings can be misleading)....check immediately after shutting engine off. on a 1..23;1 tranny, level should be on the full mark of the dip stick. 1:1 tranny ,level should be to the top of the flattened side 1/2" above full.
Just wanted to be sure your checking it right! Low fluid level can cause malfunction in just one position.

A buddy of mine suggested making sure the shaft isn't just spinning in the coupler. It's not; all that stuff is tight.

Anything else possible besides a rebuild? Any ideas why it would've happened in the first place?

Thanks guys. Unfortunately, the coupler is fine, and I think I'm looking at a rebuild. Anybody got suggestions for that?