have water entering in #3 cylinder (bottom) changed head gasket.checked for warpage, its o.k. question is if motor was run in shallow water/mud could this affect exhaust gaskets or could it be causing water to enter cylinder. i loaned my boat to my nephew (first mistake) he said it ran bad as soon as he put in. when i ran motor there was dirty water coming out of prop/exhaust. where else could water be entering ? is there a check valve? have clymers manual but does not help troubleshoot. thanks