You sure that is s vacuum line to regulator ? Maybe to send fuel back into tbi in case psi reg diaphragm ruptures/leaks. If it is hooked to vacuum and not supposed to, this can cause improper psi under different loads

THIS IS GOOD INPUT.. YOU ARE CORRECT! my current vacuum line is coming from TB and not from flame arrestor.. checked service manual and it's how you say... will try to reconnect correctly and see the difference next Monday..
Still unsure why low fuel pressure though-- Maybe the regulator got busted due to wrongly connected hoses?

who would have changed the vacuum line without your knowledge?

Purchased boat from another user 3 years ago with the engine "rebuild". Could be they did not wire vacuum hoses correctly but ironically it worked for 3 seasons this way.. TBH I always felt engine was to small for acceleration on this boat.. This summer decided to lack of power during acceleration, could be that the pump or pressure regulator got busted now..

If the engine is supposed to have 30psi fuel pressure it surely has to be a pump or regulator so need to check these on Monday.. but vacuum lines not connected in the correct order would make sense for loss of power at a different speed.

Have you ran the engine on external fuel can to check if tank issues such as pickup screen, etc.?

Yes , I tried with an an external tank , new fuel filter and fresh fuel

UPDATE:
Replaced Fuel Pressure regulator and Fue pressure is always on 30psi , it only fluctuates about +/- 3 psi . Tested boat and it feels to operate much better. Will try with Full Load next Thursday and will ensure that problem has been solved.
Only concern is if I have the vacuum tube from TB to FPR correct.. Should the vacuum line be from Flame arrestor to FPR or from TB? Tried to install it from the Flame arrestor and it makes little difference in performance.

When line is to flame arrestor, it is a vent hose, not a vacuum line. When hooked to a vacuum nipple on tbi, it will become a vacuum hose and it will reduce psi when under vacuum. As throttle blades open there will be less vacuum and psi will go up. The more throttle, the less the vac and more psi. Psi will not be higher than when hose is too atmospheric ie flame arrestor

thus my original question.

Yes it's fitted on the TB , you can even here good vacuum when on idle. So all should be good now in terms of Fuel delivery and pressure. On WOT the fuel pressure is always constant on 30psi, this means vacuum and fuel pressure regulator operating accordingly..

Will test the boat with full load next Thursday and update with resuts