On the 17... has a CompTip, check the seal below the comptip. Water can pool on top of the plug and get sucked past with the mast heating and cooling. If you need to seal it, you can drill into the mast about 12" below the Comptip and inject a two part foam (closed cell). This will create an internal plug.

If the water is getting into the CompTip, you can plug the top with the injected foam. Look at the mold part line. We currently paint over the seams (front and back, below the luff track) It was found to suck water through tiny pores in the fiberglass. We had masts filling with water that had never been IN the water. The black Hobie masts likely are the most prone to this as they heat and cool. They heat and exhaust through a leak. then cool and suck moist air in. This can happen all day long every day as clouds go over.