If it's a little water (natural condensation) you can use a product called "Heet" to help. It's either Methanol or Isopropyl Alcohol (depending on what's available and/or legal in your area) and works by drawing the water out of the fuel.
If it's a LOT of water (watered-down gas from a dishonest station, sabotage, leaking seal letting water into the tank when the car was washed) then removal of the tank, and pouring out the contents is your only option. I would also use a water removal product like Heet to clean the tank out (pour it in and slosh it around, you do NOT want to get this stuff on your hands, especially if it's the Methanol-based product, methanol is highly toxic and can be absorbed through the skin, causing severe illness and possibly blindness or death) then when the tank is re-installed, put a few gallons of FRESH gas in there, then add a bottle of Heet and fill it as soon as you can as an insurance policy against further water contamination.
I've had water in the gas of one of my Mustangs before ('87 GT 5.0, high school brat working for me at the restaraunt I managed at the time thought it was a great prank, he's lucky it didn't damage my Walbro fuel pump!) and that's how I took care of the problem. Might've been a little overkill, but I really didn't want to take ANY chances!
__________________ 1976 Ford Mustang II Ghia: 302 with a 600cfm Edelbrock carb, Edelbrock Performer 289 intake, Dynomax Blackjack headers, 2.5" exhaust with Flowmaster Super 44s. RJS 11-gallon fuel cell, C4 tranny, chrome 16" pony wheels, fuzzy dice, brown vinyl half-top, and painted in the tackiest color ever (harvest gold, that's why I call it "The Goldenrod").
Also have a 2003 Dodge Ram (lightly modded daily driver/tow rig/office/dining room/home away from home/workshop... I call it "The Big Blue Dawg".) | 
IT'S ALIVE !!
