Tinboat
If water is not getting in thru the vent or fuel lines as you have stated, then it sounds as thou water is "ponding" on top of the tank (low spot). How it gets there only you can make that discovery.
Next step would be to find where the "ponding" water is entering the tank. Either it is at the gas float switch seal or more likely it is at the back of the tank. Now let me explain: [img]/forum/images/graemlins/tongue.gif[/img]
During the original tank install, some of the foam has extended over the top of the tank forming a back wall or a containment type area. When your boat is at rest or up on plane, water is settling (since 1998 - 8 years!) in that area and finaly the fresh water has created a hole that has corroded thru your tank. Now all water "ponding" or gathering on top of the tank is mixing into your fuel.
Try lifting the tank deck cover off to find your problem
Good Luck and let us know the outcome ?