agreed on putting pressure to tank.
total pain, but the the vents and fills need to be blocked off, the pick ups too.
1 vent hose w/ a 5/8 T fitting installed w/ a ball valve so the compressor and a gauge can be installed.

turn the compressor to 5 psi, pressurize it and see what you have in the morning, if it's holding pressure,
start hooking stuff up 1 at a time until there is loss of pressure.

looks like the floor is screwed down, hope it doesn't come to that

nice place and boat, what year is it.

show me your fuel fill and with the cap off and fuel Vent(s) if they are not incorp in your fuel fill.

If you have the combo fuel fill / fuel vent then I would bet its coming in from there rain water and or boat washing.

Important you need to get all the water out of the gas tank. Raise the front and shake the boat up while sucking gas out. I know its a lot of work and gas.

keep in mind that the combo fuel fills with built in fuel vents - the vent is above the seal in the fuel fill. Water still leaks in thru the vent. Once again, I am talking about the combo fuel fill / fuel vent built in.

I have fixed many and I mean many of this style that gets a ton of water in the gas tank. Larson boats was one of the worst because they mount alot of their fuel fills / fuel vent combos exactly horizontal making it very easy for water to seep in the fuel vent then down into the tank.

The fix for these was a seperate fuel vent relocated in a more so called dryer area on the hull and using a non vented fuel fill. Prob did at least 100 of these on Larson boats alone not counting other brands..

Was on phone, and missed that paragraph.

Some things I've learned (from the "experts" here online, and some IME; often the hard way):

1) Nearly every mechanic I've asked in the marine industry has stated that it is not good to mix Ethanol fuel, and non-Ethanol fuel; that it degrades the quality of whatever you have in the tank, and the chemical compositions/bonds break down.
2) Ethanol is like a sponge, so any Ethanol in the tank WILL absorb water directly from the humid air, unless it's treated to prevent such.
3) I left my AT in Florida for a few months, years back, with Ethanol fuel in it (albeit it was NOT supposed to have been Ethanol fuel), untreated. Upon first fire up, I found out the hard way that the fuel had absorbed ALL of the Florida humidity; and ruined 4 filters/separators (literally corroded them to pieces inside) AND destroyed my fuel pumps, all due to that crap. That was an expensive lesson.
4) If possible, you may want to drain the tanks, have them vacuumed, and start fresh. This would be a good time to do a pressure check as well.
5) Star-Tron is far and away the best fuel treatment I've found, and you can buy the hi-po bottle which treats 128 gallons for about $9 bucks at WalMart. (Every other retailer I've seen sells the lo-po bottle that only treats 60 gallons, even though they are roughly the same size.)

I see now your fuel vent(s) are on the side of the boat is that correct ? I agree seal what is needed and pressure test the gas tank.

fuel water seperators filters have been on boats way before E 10 was ever at the gas pumps, with that said a known issue that water can enter the fuel / gas tanks.

Also would look at testing your gas that you buy before you even put it in your tank again. Do you ever buy marina gas ? I would test for water after you buy it no matter where it comes from.

OK, we use the chit out of this boat. I run at least 150-200 gallons of fuel a month through it. Haven't put any ethanol gas in there for at least a year...so it's all gone. Round flush mount deck fills and separate vents. I have replaced O-Rings...not the issue. Standby, will walk out back and snap a photo...

Boat is a 1999 Cigarette Café Open. We live in Cape Coral, FL

If the boat has aluminum tanks, both gas and water, then I would look at them. Fresh water will eat thru the aluminum and with water in the tank, then water will eat thru it also. That's my best guess at this point

https://cimg2.ibsrv.net/gimg/www.off...252b96ce80.jpg
https://cimg6.ibsrv.net/gimg/www.off...97cc884db5.jpg
OK, I buy my fuel at a marina a mile from my house. They have new tanks and I'm 100% sure that it's good fuel. I've gone as far as checking the tank for water just after getting fuel. The water that gets in there is happening under my canopy and while it's on the lift. This has been 100% verified. I can check it, let it sit for a few days with perfect weather, not wash it and there is a pint of water in there. The only two possibilities are that it is sucking humid air through the vents or it's condensation from temperature change. Granted, a pint of water in a 240 gallon tank isn't that much, but it's in there!!

Photos of fuel fill and vent.......The vent hoses go up for about 6-7 inches before they go down.

If the tank had a hole in it, wouldn't it leak fuel into the bilge? Never a drop in there...weird issue!!