Blocking the vent will create vacuum in the tank. At some point the vacuum in the tank will be stronger than the fuel pump vacuum. At that point the motor can not get ANY more fuel. Up to that point the fuel delivery rate of the pump will continually decrease, which will become evident by reduced max RPM. Reduced RPMs from fuel starvation runs the motor on a lean fuel mix which can, according to the shop manual, "cause serious ($$$$$) damage if left uncorrected." I presume you were out on the boat and stalled with no start (no fuel) - started looking for restrictions and discovered the tank vent was closed, opened it (corrected)& all was well again. I've done that with my V4 90HP. I did not experience any symptoms/noises of damage.
