Yamaha f115 wot from 25mph to 10mph. Why?

[Markargot1970]

We are the only owners of a 2002 Yamaha f115 4-stroke on a 2002 Alumacraft Trophy 175 boat. This has been a great motor. It has less than 1000 hours on it (best guess, no hour meter). It did not have a water/fuel separator on it when we bought it. We never bought fuel from the marina, always a busy gas station. I put high quality fuel additives in the tank every other tankful to battle ethanol. Always garage-stored with fuel stabilizer. Planned to use it more this year (2020) so I did some suggested maintenance including, new water impeller, new block anodes, new spark plugs, new onboard fuel filter, new fuel hoses including primer bulb, installed a water/fuel separator. Oil change wasn't needed yet because I did that within the last year, maybe 5 outings ago. So now it starts great. Idles a bit rough but not bad. Accelerates a bit rough. Normally I can get the boat to 30mph with just me in it. First time out after these services I got to 25mph at wot and gradually, without letting up on the throttle, it dropped to 10mph. Then I could never get it above 10mph. So I changed spark plugs again noticing that #2 and #3 looked blackened. Back out on the water I got to 22mph then, same thing back to 10mph. Check the plugs, same 2 look blackened. One coil pack goes to 2 & 3 and the other to 1 & 4. Is the 2/3 coil pack in need of replacement? This motor has never acted like this before. I'm stumped.