2004 50 HP Tracker/Mercury running rough/ water in Fuel?

[killenmike]

Last weekend I filled the pontoon a 2004 18 foot tracker with 50 HP mercury with fuel and added Stabil just like always. boat ran great for about 4 hours pulling kids on tube, cruising etc. on the way back to the ramp, it was running 3800 rpm then all of a sudden felt like we sucked up a big slug of water, engine jumping, shaking...thought it was coming apart. We were still making headway, so I slowly made my way back to the dock, ( was scared to turn it off) about the time we got there, the engine smoothed out and seemed to be running fine, cranked right up when I went to load it on the trailer. Just got through sucking ALL the fuel out of the tank, did not find any water, just some stuff that looked like coffee grounds. Now I am asking you guys, what is the best course of action. Install an inline filter, or pop for an 85 buck water separating filter and install or something I have not even thought of? My idea was to double the stabil amount and run a tank of super unleaded without ethanol through it. Any more suggestions? Could it have just been a slug of water that went on through and everything is O.K. now? Want to do the right thing to prevent this from happening again. Tank is 15 gallon plastic wedged between the toons with no way to drain from the bottom. Thank you in advance for your advice and suggestions. BTW, I did notice the needle valve that screws into the top of the cap was gone when we got back last week....Purchased a new cap at the tracker place today, could not sell me a needle valve to replace the old one...Could that needle valve being gone, let enough water into the tank to have caused the problem? This is our first year boating, and I want to learn to do it right and not get stuck far from home by doing something stupid. Thanks again.