Guys this problem has me beat, which isn't hard. I have a 1999 Mercury efi 225 on a 1999 Bumble Bee bass boat. At 5800 rpms and a 26" trophy prop, the boat use to run at 75 miles per hour during the summer months and a mile or two better in the cooler waters of the winter. But now, over a span of a year, I've lost 15 miles per hour with nothing being changed on the boat. I've replaced spark plugs but that's about it. I lost this speed at 5 mph increments and I don't know why. One day I put in and was running 75mph. A week or two later, I put in and I'm down to 70 and after a month or two, I'm at 65 and now I'm at 60. I don't have hardly any lift now. I don't know my rpm's at this speed due to the gauge not working properly after putting a new Lavorsi gauge in.
