Loss of power after 3000-4000 RPM's

Fairly new boater here.
Bought used 21 ft Monterey w/4.3 Mercruiser last year
Just over 400 hours. Boat and motor appearance in very good shape for used.

Last year first time out, Light/Alarm near throttle goes off after an hour. Brought to shop and it they disconnected and told that there was water in gas so they added a couple of bottles of SeaFoam and brought back on water with same issue. I replaced the fuel/water filter and problem went away. Never got a great answer for what the alarm was all about.

This year: after winterized boat at another shop (just because didn't want to use last shop). They changed out prop to increase rpm's and also was chewed up pretty bad. Told that boat should be ready to go for new season. Just put gas in and re-hook batteries and go. Problem was that batteries were not unhooked so makes me wonder what they did do. Brought boat on water and now runs smooth under 3000rpms (10-15mph range) but anything more shows power loss, sputtering with only half throttle. Full throttle is major sputtering and no gain in speed or rpms. I replaced fuel/water filter again but no improvement this time.

What's the deal here? Bad gas, too much water in gas tank, fouled plugs? Looking for advice on what to do next. I don't mind doing it myself if fairly easy work. Thanks in advance!

carb, TBI,MPI?

When you are running out of gas under load it suggests a fuel delivery issue. I would suggest that there is an issue with the plumbing from tank to cylinders someplace. If it were my boat, I would replace all the rubber hoses, the filter elements, and check the pump(s) output rate to see if there is anything wrong there. I'd have the tank(s) professionally cleaned out and fresh gas put in. Full tune up with plugs wires cap rotor etc
A good sign is that it is running at low rpm.
Another possible scenario is that there is still significant water in the tank. When you attempt to get up on plane the water pools under the tank pickup and you are sucking mostly water. Dunno.
Don't run at full throttle for any extended time. You are at the top of the stress-o-meter when you do that. Get up on plane then back off to around 4K or even 3500

For maximum life and minimum "Boat Bucks" limit continuous RPMs, i.e. "Cruise" to 80% of the max RPMs you see with a half full tank and normal (for you) load in the boat in "calm" water. The assumption here is that the max RPMs you see are within the RPM range as specified for that engine,i.e. listed on the decal on the flame arrestor on late model MERC engines.