In the sagas there is a great deal of description of travel by sail. In general, the boat owners stayed with the boat. If the had to leave the boat when they were away from home, they usually left people aboard with the express task of keeping the boat safe. At home boats were often hauled out of the water and kept in boat sheds (the boats were extremely valuable). There are absolutely descriptions of these boats being stolen or tampered (e.g., The Saga of Grettir the Strong).