1972 merc 800 longshaft

ok this is the problem , and what i have noticed so far, i bougt this motor and guy said it runs fine for a little bit then it runs like its fowling out,
i put it on my boat hooked fuel hose up to it and noticed it had a fuel leak! fixed the hose and then the motor seemed to run fine, ( i better add that the guy rebuilet the carbs about 5 years ago,)
i put the boat out on a lake a little to late to test motor full speed and it ran nicely at 5mph for an hour! every now and then throttling it up to see where it starts to try to plain! (it was no wake time on the lake so i had to be carefull and reserved!) the next day i took it out in saltwater to do some fishing it cruised nicely around 38 to 40 mph across the water fine , i trolled for about 20 minutes then it started the fowling out like the guy had mentioned and then dying, luckilly i made sure i had two batteries on the boat and i limped it back across the bay to the boat launch, one of the problems i seen was that the water was coming out of the plate that is where the sparkplugs go through to the cylinder, ( not coming out of cylinder but fromt eh plate around the sparkplug where you can accesst he water cavity area) I bought new plugs , the gasket for replacing the original to stop the water and i noticed the number 4 sparkplug wire was rusted to the spark plug so i bought the repair kit to replace the boot!, I am running 50 to 1 and im only running mid grade fuel since it has the best cleaning agents in it and it is more consistent octane , they never really tell you what all they put in supreme and i noticed with my car my truck and my dirtbikes that i getter fuel economy out of midgrade versous the low or the super! evan my weed waker , mower , and chain saw run better on it. Anyways i need some help trouble shooting this also a tip for romoving hte bottom two bolts on this plate! they used #3 phillips bolts versous hex bolts on this plate and hte bolts are obvioulsy corroded and seized