Had a similar thing a couple years back, vent goes out side of boat and waves were sending water into the vent. I found the vent tube was a straight shot to the tank so I added an upward loop in the vent line and then twisted the actual vent so it would be less likely to take in water. Cleaned out the tank completely and have never has an issue since. The upward loop in the vent line is a big save to keep water from running into the tank. In fact I liked the idea so well I did the same thing to my bilge pump discharge hose and I now longer take on water their as well.