If it was me I would let the boat sit long enough to ensure that any moistire has settled to the bottom(couple days?) and drain the tank. Most all of the moisture should settle to the bottom because water is heavier than gas. I would drain 90-95% of the tank and run it in my vehicle. The other 5-10% I would drain and use to burn a brush pile or get rid of. If the tank was easy to access, I would remove it and clean it dry. My friend cleaned moisture out of his tank one time buy draining all fuel, then removing the fuel intake and ran a piece of pvc with a rag attached to the end to dry out the bottom of the tank.

If that is not an option, I would drain as much as possible and change the fuel filter, then add NON ethanol fuel and see what happens.

You said your motor was a 98 150...is it a Intruder or a Ficht?