Fuses should be near the power source because the purpose is to protect the wire from burning if there is a short circiut.
If a fuse is 15 ft from the battery and the wire damaged and shorted to the negative (ground)between the battery and the fuse, then the wire is unprotected and will burn.
If you are going to run a Pos & Neg back to the battery for every circuit then the fuses should be within 7 inches of the battery. This becomes a mess of wires and fuses when you have 2 ff/gps, a VHF, running lights, stereo, horn, wipers,12v Acc socket (cig lighter)...
Often the instructions for electronics tell you to run separate wires and fuse all the way to the battery. This is for their benefit because it rules out the unknowns about your boat wiring if you call them and say it doesn't work.
If you are going to run a Pos & neg feed (8 awg) to a fuse panel under the dash then you need to protect the positive feed with a fuse at or near the battery with a 30A fuse. That panel then, in essence, becomes a power source and the separate circuits are appropraitely fused at the panel.
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