Do I need to change engine oil annually even though I use synthetic and put less than 20 hrs on the motors?
Yes. Moisture can build up and changing the oil is one way to get it out. Additionally, you can use oil changes as sort of a quick health check, by looking for undue quantity of metal, by its color, by its odor....
Change your oil according to the engine manual schedule: its cheap insurance.
Kevins right also when engines burn fossel fuel byproducts are left behind,and those byproducts get washed into the engine oil,and it also contains acids, which over time will eat away at the babbit material on the bearings inside the engine so before you store your boat away for the off season change the oil, as Kevin said "It's cheap insurance"