i have had bad manifold and water in the oil have the manifold checked could answer all your problems

I just checked the manifold with the acetone trick...No leaks that I can see. However my cousin that took off the riser off the manifold on the side of the motor that had the water in the cylinders just brought something to my attention. He noticed that when he took the riser off the bolts were loose. Like more the finger tight..but not much more then a nudge with a wrench to loosen them. So here is what we are thinking now, and let me know if anyone thinks this would be possible. That when the boat is running the water that is coming into the manifold that comes up and pushes on that gasket before it exits through the little crossover tube into the riser, could it be pushing past that gasket and running down into the manifold??

I run EMI Thunder Exhaust on my Big Block's, the only thing going past the Riser Gasket is Exhaust, not water, you may have a cracked exhaust manifold or riser.

Ya I know that..but did you read the part were when we took off the riser the bolts were loose..

what style emi s do you have ???the old style the top of ex manifold where the 4 bolts or studs are was orig open to pass water to there aluminum style risers .if you had the ss style it blocked the top off and you ran the little hose instead ..ive had that old style leak many times
my cure was to weld all the holes on the top side up and surface it .never had a problem again ... it was such a problem emi now makes them closed off .was a very bad desighn

I have the ones with the ss risers. It has the 4 bolts on the top holding the riser onto the manifold. It has the gasket that blocks off the 4 oval shaped holes...which leads me to believe that since the bolts were loose it was causeing the water to leak past the gasket into the manifold..

sounds like you are on to something

yup theres your problem seen it more than once ..weld them up and surface them ..those are the old style manifolds
your not the first person to pull a motor down for this

emi had so many problems with them that now they come without the water passages ..ive welded many of them up resurface the top and your done

I have the SS Risers, Mine do Not have any water passages, I must have the new style