Hi and firstly welcome to the forum
Let's start wotha few more bits of info please, does it sound like its misfiring on one or more cylinders? When was it last given a good service and had the fuel filter changed?
It could be many things, but let's use the basics you need fuel air and a spark to make it work, so thinking of fuel - presume you have some and air vents are clear and the fuel filter is newish, is the fuel good or old and may have gone off or gummed up the fuel lines/ filters. Air - is the flame arestor clean and the carbs seem to be working ok? What about spark, are all the leads in good condition and attached properly, is the rotor arm clean?
It sounds like its suffering under load, so is it not getting enough of the three things to meet the demand, try testing each in turn. Also you say that its been in freshwater, have you just started using it in the sea? If so the wave bouncing could have dislodged some gunk in your fuel tank that could have wrked its way througha fuel line to either a fiter or block a line partially.
The above is just my basic starter for 10, I'm sure others will post more later!
Have a look at the op manual and the trooubleshooting section, it will give you a basic checklist like the above to heck out.
Hope this helps, where are from and where do you go boating?