Mercruiser 43 ltr stalling when attempting to accelerate

Ive got a 4.3 ltr 185 hp Mercu

Ive got a 4.3 ltr 185 hp Mercuiser I/O with an Alpha leg. Just tuned it up to use it for the first time this year(sadly). It ran fine in the driveway so we took it up to the lake for a day of fun with friends. It started and idled in gear fine but abruptly stalled when trying to accelerate. We ended up having a nice picnic on the beach. Any ideas?

I forgot to mention it runs an

I forgot to mention it runs and revs up fine when not in gear so I think tthat eliminates carb or fuel problems. My initial though was throttle cable adjustment which I will work on today.

If it was fine the last time y

If it was fine the last time you ran it the cable is not the problem.
Unless it was change since you last used it.

Just about every engine will run or seem to run fine with no load on it. All it needs to do is spin after all.
This kind of sounds like a dirty fuel system as in the main jets and the carb need cleaning / rebuilt unless it is an EFI model.

If it is EFI just take it in and have it checked.
I found here that the EFI system is not something that someone with limited experience and a lack of specialized tools should be trying to fix on their own.

Unless you know how to repair a carburetor / EFI system take it in.
It may not be rocket science but if you screw it up it may look like a rocket.