It is important to clarify if this is a backfire or a misfire. Backfire leads you to fuel first and misfire leads you to ignition first.
A lean fuel mixture takes longer to burn and therefore causes a backfire. Running lean will cause a loss in power and it will also cause a rise in temperature as it burns hotter, which you might confirm by looking at your temp guage.
A misfire can be caused by water in the fuel, low ignition voltage/poor ignition system performance - too rich fuel mixture - and a few other things that we don't need to think about yet.
If this was my boat, I would whip out a spark plug and see what it looks like. If it is carbon fouled (black powdery) or wet - look at the ignition system. If coil voltage, points dwell, wires, plugs & timing all check out then turn your attention to the carb.
Take off the top cover and look in the bottom of the float bowl for water, verify float level height and give it a good clean with carb cleaner. Also, while you are in there take a look at the float needle tip and see that it is in good shape. If you make a float adjustment then reset the idle pilot screws 1.5 to 2 turns, then tune at operating temperature.
If the plugs looks normal or maybe a little lean but are dry, focus on fuel delivery as your ignition system is not likely the culprit.
BTW, you need to run the boat in the "bogging" condition for a couple minutes before you pull the plugs to look at them.
