Water is heavier than fuel, so "sinks" to the bottom of the tank.
If you have an actual drain or outlet on the bottom of the tank, you can probably get most/all the water out. If your fuel pick-up is at the top of the tank, the suction tube usually never goes all the way to the bottom of the tank.
If you have an inspection
port on the top of the tank, you can remove it and stick a hose down to the very bottom of the tank and suck up the water.
Hopefully we are talking about a diesel fuel tank
is pretty safe to work with but gasoline is really, really dangerous.
Once most of the water seems to be out, rock the boat a few times while draining and you may be able to suck out any remaining amounts. Then just run the engine
and check the Racor for water and change as found necessary.
If your fuel tank is not made out of marine
, the puddles of water sitting on the bottom of the tank will eventually corrode through. I speak from experience....