BINGO ! big breakthrough yesterday , I had a real good look at the wiring compared to the wiring diagram, the plugs on the ECM are consistent with MEFI 3 . the wire going to the shift interupt plug on the harness is purple not white, my boat does not seem to have a shift intrupt ? ( is it on the gear shft plate on the manifold?) I tried putting a jumper wire between the blk and pur on the loom plug that is not plugged in and it immediatly cures the start then stop with MIL alarm problem!
I tried without it again to be sure and the problem returns .
It starst and idles great now so i put it in the water , it pulls good up to about half revs but then holds back if the throttle is opened more it stalls but if backed off a little it runs smooth . I ran it around for 20 mins at low throttle . Its like fuel starvation symtoms.
I drained the tank before as its been standing for 3 yrs and put in 100 liters of gasohol 95.

I am confused about the model ? am i correct in thinking that plate number is missing a zero on the beginning but shows the block is 5.0 liter? It does not have closed loop cooling. The previous owner and the 'mechanic' both say the replacement engine was supplied complete from Bangkok with ALL electrics manifolds etc . but that was one and a half years ago ! it has been standing for that time as no one could get it to run !

are there any checks I can do to see if the injection systemand ECM are matched with that longblock?

any help on the dying problem would be great thank you for your interest

It does not

Need to check your fuel pressure and should be 43 psi or 3 BAR

I am getting hold of a fuel prussure tester today from the boatyard , could i just clarify what the fuel pressure should be? It says 30 PSI in the manual ,what RPM should it be tested for reference. do I tee off right by the injector body ? sorry to ask but I cant find much about this test.
It seems my motor is a 5.0L 1998 TBI MEFI 3

Yes correct 30 psi807952A1 Mallory 9-33205 (30 psi)

Fuel pressure is 34 psi , cleaned out the tank, new filter. ( the old filter had a tiny bit of dirt in it and a few drops of water only when i rinsed it into a clear container) While the test pipe was connected i removed the pressure gauge and put the end of the pipe in a can , it still idled perfectly and filled a 5 liter can very quickly . so I guess i have not got flow problems ?
It sarts and idles perfectly and revs clean on the flush hose but in the water it is ok up to half throttle but then on opening the throttle more it dies away , close the throttle and it picks up again . the symptoms are EXACTLY like fuel starvation .
What else could cause this ?

Look at the fuel psi gauge at that failing rpm/throttle position.

Use a spark tester if it has a distributor on the coil wire then bring the rpm up and see if spark goes weak?

What would be the likely cause of the spark going weak? the coil?