This is the light we would like to hang from the bow at times.
Also am considering replacing bulb with LED.
Yes there is a ground wire for the return. It also runs in yes you are right parallel.
So just add them up and put in a 25% larger fuse. Where do you get the numbers to add up? Off the bulbs?
Yeah, sometimes it's hard to find.
The LEDs are insignificant in terms of amperage draw....very, very little. The incandescents could be as much as, say, 25W or a little over 2 amps each. Say, 5 amps for the two incandescents and another amp or so for the LEDs. A 7A fuse should be more than enough.
Note that the adequacy of wire size depends not only on amperage draw, but also on the total length of the circuit. For lights and non-critical equipment, a 10% voltage drop is considered OK. For critical equipment, a 3% drop is preferable, or even less. There are lots of voltage drop tables on the Web, and in West Marine Catalogs. You can do the math yourself.
For what you describe, AWG16 should be OK for runs of 20' or less, round trip. If it's much more than that, you should use AWG14.