115 Mercury Optimax losing power

[Matt112604]

Hey everyone, in dire need of some help here. I have a 2006 bass tracker tournament 18 with a 115 Mercury Optimax that i bought used last year from a friend it has very low hours boat ran awesome until i had an incident where it ended up slightly submerged and got some water into the fuel.... I brought the boat to a shop they siphoned all the gas out( as much as they could but im sure they didnt get every last drop) and they went through the fuel system got it running good again but a year later and im still having issues. There must have still been some residual water in the tank that is still giving me issues, so i think. The boat will be running great and then i will start to lose RPM's slowly, it will start to spit and sputter at 5000 so ill give it less throttle to keep it running good and then i will slowly start to lose a couple hundred RPM'S at a time as it gets worse and worse until eventually i can only cruise at about 7 MPH without the boat spitting and sputtering. If this happens i will change the water/fuel seperator take it back out on the water and it will run great again for a certain period of time which tends to vary, i thought i had it finally cleared up after getting 40-50 good miles out of it running flawlessly but i hit a large boat wake causing the boat to bounce quite a bit and then it acted up again almost like it sloshed the gas around enough to stir up some water that is still in the tank, this has seemed to be the common theme making it act up any sharp turns or getting in rough water causing the fuel in the tank to slosh around quite a bit. I empty the VST tank and sometimes i will have a tiny bit of water come out and sometimes i cant see any, so i started to think maybe this is an issue not related to water but i had the boat running great for a while now and as soon as i hit a big boat wake last weekend it acted up, changed the filter and yesterday it was running great again for 8-10 miles until i hit some rough water and started bouncing then it started to run bad again so it really seems like water still but i keep changing the filters and running k-100 in it i would think that would fix the problem. So if it is water is there anything else i can try to do to clear it up? Or do i have an issue that is something else? Any help would be greatly appreciated, thank you.