your over complicating it. your bow light has 2 wires, your stern light has 2 wires. your switch has 3 wires.
all marine lighting comes with both leads. chassis ground is a no no in marine wiring. it will dissolve your aluminum boat in a matter of months (electrolysis)
your switch has 3 positions, off, stern light only, and stern light AND bow light. your switch is only switching the 12 volt side (not the ground)
from your battery, 2-wires of adequate size for your lights (16 gauge should be fine) to a fuse and terminal strip (12 volt is fused, ground goes to common terminal strip)
from the fuse, a wire to the common terminal on the switch
from the switch, one remaining wire to the bow light, one remaining wire to the stern light
the extra wires on each the bow and stern return to the ground terminal switch.
since your light switch has 3 wires, and one wire is a different color, that wire will be the common (goes to the fuse)
simply google navigation light wiring if you are still confused.