Just checked - my Merc 5hp does output DC and is connected directly to the battery. It was my previous boat (again an old Merc) that was AC!. Like yours, at idle/low speeds, it's around 4V and only gets up to 12V at quite high revs. I hadn't realised that!
I also have a solar panel which trickle charges the battery. This is generally enough to keep the wind, log and GPS going, but if I use the nav or cabin lights, I soon need to charge the battery.
Geoff
Parker 21 / 18 Dawn