you could have the vent coming up through the deck next to a stanchion or the house. With a loop at the top and screened to keep out the vermin. Then it is in sight as you fill your water tank. This is the way it is done on work boats. Then when we have an overflow we know that the tank is pressed up.
But what ever you do, DO NOT combine the fuel vent with the Water vent... A definite NO NO!