Did you check the Heads for Warpage, and Resurface if necessary?
The entire Gasket is not Rubber/neoprene/Silicone, it is usually Plastic, with Rubber/neoprene/Silicone Rings around any Ports.
If the Block-Heads were machined, then Holes in the Gasket may no longer match the Ports on the Intake.
An Acquaintance acquired a Boat and Motor, as part of a Trade. "Runs Great!", he was told. He was convinced he needed Head Gaskets, even after I took this Pic. Actually he did need new Gaskets, as well as a new Block and Heads
Since you have the motor out of the Boat, I agree with Scott, that it has to come apart for a very thorough Inspection.