You may as well just drain it all and replace it, if you let it sit for a day in containers without moving it ypu can salvage most of the fuel mix by pumpimng it off the top of the containers
To remove any remaining water from tank just add a cup of methylated spirits, slosh this around and drain, let it dry ,the water should mix with the metho and dry very quickly.
I found out I had water in the tank AFTER I topped it off. I used an old bulb and gas line to suck out as much as I could(internal 20 gal tank) then installed a gas/water filter which removed the rest. No problem since. I'm a big fan of the filter.
How big of a tank are you talking about? If it is a large tank, you will need to get it an angle where the water will settle to one corner and siphon off the "bottom to get the water out.
