take out all three bulbs, and then flip the switch, see if the fuse blows.
Then, install one at a time, until the fuse pops.
since the boat in fiberglass, I bet the short is in the switch on the helm.
My anchor light is jumped to the nav lights, so your cannot just have the anchor light on.
I'd bet you have two positives, and one is grounding to the negative, as the switches are in series.
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