Is it leaking at/near the sink itself? Is it actually the hose that is leaking or a fitting? Originally that line, as all water lines on the boat, would have been copper tubing. If it's hose, then somebody replaced it, most likely because they installed something new or removed something that was originally there, like the oem water filter.
I have to admit that I don't know the actual routing of that water line though if I had to guess, I'd say it probably runs above the headliner in the master or guest stateroom and then up through the aft deck floor. It could run inside/along the base of the paneling in the salon but that seems very unlikely to me based on the way most water lines are run on the boat because it would have to turn the corners at the walls. But this is just guessing on my part.