Paul-
I have a friend who recently bought a Honda EU2000i for ham radio power use, he's quite happy with the 12VDC output for charging deep cycle batteries. And, the AC output is incredibly clean for a genset, see
http://www.jkovach.net/projects/powerquality/ for some photos of scope tracings.
But a gasoline genset on a small boat is a danger. Once from the gasoline, a second time from the monoxide in the exhaust and I'd be more concerned about that.
I'd opt for the genset if AC and "power now" was the need...but if possible I'd prefer solar panels and just waiting it out until the starting battery was OK. (I've used a 12V 17Ah gel/AGM cell to start an 18hp diesel, I'd guess it will start a 50hp outboard just as well if there's no pull start hidden in there.
Didn't think it would work, but that's what is inside a lot of those "jump" boxes.)
The problem with solar panels is derating the output reliably...in medium latitudes and mixed wx, 25-30% of the rated output for six hours is about all I'd expect on any given day. (Better in June, worse in February, etc.)
A wind or water generator might be a better idea, certainly higher power output per hour for the initial cost.