Off Topic: Waverunner Engines Tech Question

Just bought my first waverunner since I am now 15 minutes from the Gulf Of Mexico. Question is I bought the Honda because of superior build quality but they do not recommend synthetic oils. This is a 4 stroke motor and I assume that after breaking in this engine I should switch to FULL SYNTHETIC. The dealer is warning me against this. Anyone into these motorcycles on water?
I would like to get several unbiased opinions on it. Certainly seems to be just common sense that going with synthetic is the right way to go but they were so against it at the dealer!!