Fuses or circuit breakers are to protect the wire(in case of a short circuit), its not so much that the wire needs protection, but if it gets to hot and melts the insulation, it becomes a dangerous fire hazard. SO the wire needs to have amperage appropriate fuse or c.b., you can have one smaller, if you also want to add some protection to devices on that circuit. There are lots of kinds of wire, which vary in their ratings and how they are routed is also a consideration, one wire in open air has a lot higher rating than a bunch crammed in a wireway or tied together. Also in a boat, I would always opt on the side of overkill, heavier wire is physically stronger wire and fire on a boat is a really bad deal. As a very general rule, if you have 20 amp fuses or circuit breakers(I would only use the manual test/reset kind), then a marine grade #14 wire should be fine. Always use crimped or soldered and taped or heat shruck connections on a boats, never wire nuts or simply twisted together and secure the wires so they are rubbing or moving around.