250 XS not making power

[ZO6]

2005 250 XS OT999026 Changed fuel filters and spark plugs prior to this trip to FL. Ran great ... top speed 72.6.. 5650 rpm. Have had my boat in the water for the last 2 mos. and using every day. Out several times for premium gas and cleanup. Has started and run perfectly with a couple of exceptions. Very slow to get on plane on one occasion and on several occasions speed change(rpm) without moving throttle at about 25mph.
Been fishing in a speed limit restricted area. 25 mph Intercoastal waterway and idle on and off flats. Yesterday, I cheated and opened it up for about 1/2 mi. Top speed 50 mph 5000 rpm WOT. Today the same. No misses, hesitation, noises. Tomorrow..... change fuel filters, plugs. Any other ideas? Don't want to get started with a Merc dealer. Going home to Arkansas on Sunday.

[EuropeanAM]

I'd look at spark plugs, plug wires, and if you have a timing light available.... check for spark on all six cylinders.

You can simply clip the inductive pickup lead to each plug wire, while watching for the light to flash.

From your description... it sounds like you're down a cylinder.

Beyond that.... the remaining basics:

-Compression
-Fuel Quality, Fuel/Air Pressures & Leakdown, Fuel Delivery.