It is permeation of the fuel tanks from the fuel. The plastic tanks vent through the side walls. There is minimum amount of permeation through the cell structure. The tanks may be leaking through a gasket on top of the fuel sender, though it often is just permeation. This began to get worse about 5 years ago. It may be fuel additives and it may be other issues. Take a look for a small leaking gasket or hose, though we think it is more than likely permeation. Though give your boat a once over and check it well for actual gasoline to be sure it is not a leak. We would like to know your year, model and fuel capacity.BCB