If the pump is running every 15 min., how much incoming water
does that represent? Seems to me, like such ingress would be pretty easy to see when you find it. If the pump is actually pumping water out of the boat, it's working a lot, and your concern about failure gets more likely to become a factor. Float switches are not forever, either.
I suppose it's possible, if you filled fresh water tanks
prior to going out, that you might have a leak somewhere in the fresh water system, cracked or pitted tank, broken hose clamps, split hose, that sort of thing.
Good luck with it, I hope you find the source soon.