Being that you should change the oil every 50 hours, or every year as you winterize the engine (so that it has fresh, clean oil in it while it sits in the cold for a couple months), you won't realize the benefits of using synthetic. The primary benefit of synthetic is that it lasts much longer and runs cleaner. But since you live up in a colder clime (I'm assuming that's where your boat is also), you'll want to change it for winterization.
I would just stick with the manufacturer's (and Moyer Marine's) recommendation and use plain old 30-weight.
Someone recently posted a poll
over at the Moyer's Atomic 4 forum and nearly 55% indicated they use 30 weight. Only 2% said they use synthetic. Not saying that just because everyone else does it that's a reason to do it; just a data point to consider.
Here's what Moyer has to say about oil choice: Moyer Marine Atomic 4 Engine Rebuilding and Parts
Note that if your engine is in a typical installation
, you're not getting all of the old oil out anyhow, so even you put synthetic in, you're only replacing about 60-70% of what's in the crankcase.