Hi, water has a higher density than fuel, so it should sit at the bottom. If it's a lot of water, I would disconnect the engine from the fuel line and drain the tank via the fuel line. There will still be some liquid left which is below the level of the sucking pipe.
Then, a reasonably installed tank should have a "man hole" or any other hole (sounder access etc.) somewhere on top. Open it and suck out the remaining liquid with a flexible tube.
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