I dont know how close to the water you are but i would just do the plugs first drop the boat in the water and check it out use the seafoam as a last resort its a 2003 and if you have never done the plugs i would say it is time
my 30 hp yammy although a bit older was made to run at 100 to 1 mix is this the same for yours it to was a 3 cylinder you might want to check into this
as far as a fuel filter take a picture and go to car quest and match one up they are all the same pretty much and it is cheaper than a marine specialty an impeller can be done in a couple of hours even change the oil in the lower end in that time