Meths and Isopropyl alcohol absorb water which is why they are often used to clean fuel tanks. They are also solvents. Not sure why you wouldn't use either.

If your taking the tank out cant you just flush it out with a bit of petrol. Slosh a litre aroung and tip out a few times. Meths does absorb water but If I remember right it only absorbs a small amount. Something like 1 litre will absorb 5 mls and as above what about the salt.  

I had exactly the same problem after fuel cap wasnt screwed fully down onto its seat on a Haines Hunter 650.

I used a small scavenger pump on the fuel line to suck out as much as i could then added some meths and small amount of petrol, and sucked out again, finally removed fuel filler line and dropped suction hose into back corner of tank, recovered about 2 L of water all up.

my underfloor 200L tank (AMF) was full of all sorts of unpleasant crap when i got the boat causing lots of flow issues. Pulled it out, took it to washworld and rinsed it with teh engine cleaner/degreaser then high pressure water rinse a few times, then took it home, put my hotair paint stripper gun on low heat against the inlet with all other connections open for a while. tehn put 10L of petrol in sloshed around/inverted tank etc tehn tipped gas out inot a clean bowl and inspected, still dirt and water. filtered teh petrol with fine cloth, put it back in, repeated this till teh petrol coming out was clean. reinstalled tank, put 50L gas in, about 250ml of meths and went out, ran fine and has done ever since. I couldnt find anyone commercially to help nor any other useful info hence the possibly unusual approach!