You clearly know a lot about wiring, certainly more than I, and more than enough to confuse me. Especially since we are talking in generalities over time-lagged forum postings.
But marine-grade wire is tinned AWG, not SAE plain copper. And boats move around a lot, and flex, and are subject to vibration and weather, and seem more like a sidecar rig to me than they do to a house.
Re fuses - most of the gear I've seen has a fuse with an amperage rating higher than the actual current draw of the attached device. Heated gear is a good example. So is a GPS. And Aux lighting. And Auxiliary power plugs. Seems like the fuse is there to protect the wiring rather than the device.
I understand that I should not attach a 15-amp device to a 22-gauge wire with a 5-amp fuse. But I don't see anything wrong with attaching a 5-amp device to a 16-gauge wire with a 15-amp fuse.
Confusing matters further is that my GS runs on a CANBUS and thus has no fuses except for those various owners have put on. I haven't the foggiest idea on how to figure out how BMW has 'fused' the OEM wiring, or what effect tapping relay trigger wires into that wiring might do. But we all still do tap into the OEM wiring for LOTS of things - and the roadside is not littered with burnt out hulks.
Maybe if we can bring the conversation down to specific applications, rather than generalities, I can become less confused.
It is better to have ridden and crashed, than to have never ridden at all.