Couple of things. First, water will float, so if you have a good amount of gas, you'll be ok. There are things like HEET that will absorb some water, you can always toss a bottle of that in if you're concerned. Unless it's A LOT of water, you'll be ok.
If it were me, I'd drop a bottle of HEET in it now and fill up before it hits 1/4 tank. Maybe throw a bottle in every fill up for a few times.