Once you find out where the water is coming from you can clean out the tank yourself. Here's how:
Pull the fuel sender from the tank. Once you do that you can shine a strong flashlight down in the hole for the sender and see what's in the bottom of the tank, especially if the fuel level is fairly low. Now you get some clear plastic tubing that is resistant to gas (I forget which kind of plastic that is - maybe vinyl?) in a size that will fit on a outboard primer bulb. You'll probably need about 10-15 feet of the clear plastic line. You also need a small wooden dowel (1/4 inch diameter, about 2 feet long. You want to attach several feet of the plastic tubing to the intake end of the primer bulb and use cable ties to attach the dowel to the tubing on the intake end. Then attach the rest of the tubing to the discharge end. Now you start a siphon going by pumping the primer bulb. Keep the whole run of the siphon tube as low as possible to help get the gas over the high point and get the siphon started. Put the discharge end in a gas can and start siphoning. You can then aim the pickup end of the plastic tube, using the dowel, to pickup water on the bottom of the tank, debris, and any other crap in the tank. You can actually just siphon all the water and crap off the bottom and use the rest of the gas, but I figure once you've started this you might as well get that tank as clean as new!!!
And you literally can do that. I've done this on several boats. If you take your time, you can get the inside of that tank clean enough to eat off. END of gas problems!
A couple more things to note: You need a way to get rid of all that gas. Probably the best way is to have a truck with a large tank that is nearly empty. Water will sink to the bottom of the gas can you are siphoning into, and debris can be filtered out with a coffee filter in a funnel. Have several gas cans ready and figure out how much gas you'll have to get rid of.
The other thing is to seal the sender hole well with a gas resistant gasket. There are special types of red rubbery gasket material that will work. You can also use Aviation Form-a-Gasket sealant liquid Item number 765-1210 available from Napa.