Does the gas have ethanol in it? If so, you can get phase separation; if there is enough water in the tank to combine with ALL the ethanol, you can lose up to 3 points in octane!

I believe some of the "dry gas" stuff is just ethanol, so that may not help.
I think you'll have to keep draining the water out of your separator/filter. One thing I did that you might consider if boat is kept on trailer: install a tee downstream of filter with a portable fuel tank male connector fitting on it. Then rig up a siphon line with a female fitting and primer bulb on one end. Run one end of hose to a 5 gallon jug, plug into fitting, and pump the primer bulb till fuel starts flowing into jug. Drain the gas out into jugs and check filter for water after each 5 gallons until you don't get any more water. If the gas was 87 octane to start with, I'd think twice about using it because it could now be 84 octane, and that can kill an engine quickly, due to detonation/pre-ignition! Some octane booster is also just ethanol, so you have to be careful with that stuff too, as you won't get the expected octane boost if there is still water in the tank. Better to buy at least 90 octane gas until you're sure all the water is gone.
On a related note, on my recent 700 mile trip around Fl. that I posted some pictures of a couple months ago, I made a habit of draining some fuel out of the Racor after each refueling. I bought gas at marinas 8 times and found water 5 times! Fortunately it was all non-ethanol gas, at least 90 octane, and now I know why . . . if you buy gas from a marina, it's more likely than not got some water in it, due to the low volume of fuel sold and damp environment!

Not a big deal if it's non-ethanol gas, as a good water separating filter will take care of that. If a marina is selling ethanol gas however, it could be a VERY serious problem! Ironically, I've never had any problems with water in the fuel from a high volume gas station, but that doesn't mean it can't happen. Lesson learned is to check the filter for water after each refueling, just like private pilots do!