A ship will only move if there's someone on it or it's getting pushed by a player or a creature. Sometimes wave action can cause it to move if it's in shallow water but it won't float off to sea by this.

Best to stand in it and whack the mast IMO.

I had a Karve parked for over 200 in game days on the dedi server I played on...it never moved other than up an down with the waves. It was protected by an earthen pier on two sides so creatures couldn't get to it. A troll did get to it once and did it some damage but he was killed for his actions and the Karve repaired.