There is a principle to remember here which is that the current a battery will accept decreases at it charges up. The last 10-20% or so takes an hour + regardless of your charging source. This is what makes the solar panel great for a mooring boat.
Even if you run the engine enough to supply plenty of energy on paper it won't run long enough to top it off. So even your lightly used battery will spend all season at 60%-80% instead of 100%. Not a huge deal but it decreases the life of the battery. Even a 2W or 5W solar panel, I have this one, could probably keep your bank at 100% all season.
I literally just throw this panel on the cockpit seat and plug it into the cockpit lighter outlet when I leave the boat (the outlet has a diode wired for this purpose). So far it's kept my group 27 and group 24 at a good 12.5V.