The Durmstrang ship is a magical vessel which can travel between bodies of water, regardless of whether they are physically connected. Before the 1994-5 Triwizard Tournament, the students from Durmstrang arrived at Hogwarts aboard a magical ship which surfaced in the centre of the Lake. “It had a strangely skeletal look about it, as though it were a resurrected wreck, and the dim, misty lights shimmering at its portholes looked like ghostly eyes” (GF15).
Underwater ships are mentioned in question 11 of the first W.O.M.B.A.T. test and may be a very serious breach of the Statute of Secrecy (JKR).