I had the same problem with a 40 Yamma a few years ago. I had to drain the tank and put in fresh fuel. I also had to drain the bowls. There was a screw on the bottom, unscrew and the water will drain out. To prevent this from happening again I put in a water separator/ filter in line and it traps the water. I also found out with boat motors and tractors that it is best to leave the tanks full if it is going to be idle for a long time.

Good luck.