Re: gas tank problems, water, vent
OK...I think I see where you are coming from. I was thinking he has a Thru hull fuel fill fitting. His vent is either integral with that or separate from it.
Maybe the OP can give us this info.
I agree!
And I am HOPING he has has a Thru Hull Fuel Fill.

I have also never heard of a Under Deck Tank with a Vented cap at the tank.
And I never want to see one either!
The OP should be back soon with an update on the vent.
OP,
The Fill line will be a hose about the diameter of your wrist.
The Vent line will be a hose about the diameter of your thumb.
It will be part of the Fill Fitting or a separate fitting right next to it.
Look on the inside of the Fill fitting, any hose about 3/4" in diameter close by is going to be the vent line.
Trace it out. it should not be kinked of pinched anywhere.
As stated before, Insects may like to make a home in the opening.
Open the Fill Fitting Cover, you should be able to blow into the Vent line easily.
(Did I get all that Nomenclature correct?)