Stupid question but what did your old spark plugs look like when you pulled them out? Also, check your lines and make sure you have no water in your fuel or that there is no fuel or vacuum leaks any where.
I am familiar with your model of stand up ski but I do not remember, do you have to premix the oil or is it injected? That could also be a factor is a bad mix or your oil injection could be malfunctioning.They had grey/brown stuff all over the sparking area. They are the third set I have put it in two weeks. I did put lower grade mixing oil in the fuel