Have you dropped and dumped your Yamaha filter cans to see what you have? By the time your engine alarm goes off they are mostly filled with water and pass it thru to the secondary filter on the engine. You need to be careful you don't damage your engines.
Highly unlikely your getting water into the tank via the sending unit. If so you would be leaking fuel when the tanks are full or when you put the boat up on plane and the fuel shifts to the back of the tank. Not enough water comes thru the deck plates anyway.
Fuel fill gasket? Maybe. Easy to try. Even if it is the problem you still have to get all the water out of the tanks.
In the meantime while your trying to figure it out I would try a Sierra clear bowl filter with drain or drop the cans and dump them before and after every trip to see what is in them.
My opinion is your not getting all the water out of the tanks. Work the filters hard and it will clean up.
I assume you are running pure gasoline not ethanol blend. If ethanol it could be phase separation.
Getting rid of 50 gallons of fuel will not be easy. Clean it up and burn it in the boat. Do not put it in your car/truck. Boats have better filtration and water seperators to handle bad fuel.
From a previous post:
I have a 1997 268 Islander. Couple of things I can tell you to help.
I regularly bring my boat back from a winter trip to FL and pump some ethanol free fuel out for my lawn equipment. I use a automotive fuel pump with a long power cord that plugs into my truck (keep sparks away). Hose is hooked up to fuel bulb, prime it and start pump. Fuel goes into 5 gallon fuel cans.
Underneath the deck ports I use a 18" x 18" sheet of polly to cover the business end of the fuel tank. If ports leak at least it is deflected away from the fittings.
That "goo" that you see may be the adhesive from the plywood inside your deck hatch covering the fuel tank. Mine was compromised leaking a coffee colored liquid on my deck when removed it. Tanks had sticky glue like mix on top of them.
Fuel compartment doesn't drain into bilge. There is a glassed in hose running from the front of the boat to the back. The idea is to keep leaked fuel in the fuel compartment. At the start of the project I had pumped the liquid out of the compartment (plastic hand pump) to check for fuel (there was none). After I cleaned my tanks I pumped all the water out and vacuumed out what was left.
If I were going to wash my tanks I would pull the deck hatch, clean them up and pump all the water out so the tanks are not sitting in water that has no way to drain (only evaporate).
