Most ships have water-circulating systems that keep the engines cool and run the A/C and other environmental systems. “Raw water” or water from the sea or lake is sucked into the ship via “through-hulls” or one-way vents under the ship that bring the water in. The water then is used for a variety of purposes (like those mentioned above) and then pumped out of those small holes that are constantly releasing water.
It's probably exhaust from the water cooled marine engine, though, so please don't worry :)
No problem, have a great time sailing :)