You don't have to drain the whole tank. Water is heavier than gas, so by default most of it is at the lowest point in your tank - where the pump pickup is. That's why it runs like crap, you are running high concentrations of water with every volume of "gas" the pump puts into the engine.
Lift the tank a little, disconnect the fuel line and get a tall container in your hand (I have used a tall plastic cup before). Aim it at the outlet of the pump and turn the ignition on. The pump will "prime" the injectors, pushing the water/gas mixture out into your container. Repeat this several times until you see clear gasoline coming out of the tank. Reconnect everything and go for a ride.
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