The vents are there to ventilate the bilge in case you have a fuel leak, whether from a corroded tank, leaking sending unit, hose, etc. Obviously critical on an I/O or inboard, for which vents are a USCG requirement. Early Potter literature also claimed that it was required because they didn't foam in the fuel tanks, but don't know how true that is. (Foaming in fuel tanks is always a bad idea because wet foam helps the tank corrode faster, so one more data point on why SeaCrafts were built right!) One other advantage to a ventilated bilge in addition to reducing rot tendency, is less mold/mildew and odor accumulation below decks.
If it were my boat, I would NOT close up the vents!
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