What is that ship and why it was drowned at the end?

Question

At the end of Harry Potter and the Goblet of Fire (2005), this ship is shown sailing and apparently foundering:

What is that ship, and why it was drowned at the end?

Answer 1

It's the ship used by the delegation from Durmstrang (Igor Karkaroff, Viktor Krum, et al) to travel to Hogwarts. As they arrive, it is seen surfacing from under the lake, so its submergence at the end is just the ship returning along the same path as it arrived.

I'm not clear why it travels under water, but then, it's all part of the magical world.

Consider that the delegation from Beauxbatons traveled by flying coach (as can be seen in the same shot as the submerging ship). It's an interesting juxtaposition as the two visiting teams seem to possess diametrically opposite attributes and qualities. For example: Beauxbatons is all girls where Durmstrang is all boys; Beauxbatons enters the great hall in an almost "angelic" manner, where Durmstrang's entrance could almost be described as "demonic" by comparison, so the ship traveling under water neatly counterpoints the flying coach.

Answer 2

This is the ship used by the students and faculty from Durmstrang Institute. Once the Triwizard Cup competition and festivities are completed, it departs, carrying the students back to their own school.

As to why it travels to and from Hogwarts underwater, that is a cultural allusion. Durmstrang is apparently located somewhere in continental Europe, although the precise site is not revealed. The ship, by traveling underwater to the school, appears to preserve this mystery and uncertainty. Probably even more importantly, however, the name "Durmstrang" is based on the German phrase, "Sturm und Drang," which, translated literally, means "storm and stress." It is also a conventional expression, in German and even in English, for turmoil—particularly cultural turmoil. The dark, German associations of the "Durmstrang" name are alluded to by having the Durmstrang Institute ship go underwater, like a submarine. Because of the key roles German U-boats played in the First and Second World Wars, submarines have become the most quintessentially German of all modern seagoing vessels, and the film is playing on this cultural association.