Properly designed, there should be little risk of salt water intrusion of your fresh water tanks from the vent.
Having said that, our last two boats had water tank vents inside the hull, high up under the hull deck joint in one case, and high up in the head compartment in our current boat. Neither set up caused us any trouble either. (and it is one less fitting piecing the hull skin)
One downside though; many people like to start to fill their tanks and walk away, shutting the water off when the fill line overflows - this is not recommeded practice in our case as the vents are slightly lower that the fill port and inside the boat.
I imagine that's the reason most are installed overboard. or in the cockpit area.