Let us note the way in which the barge is presented in the text itself as it mysteriously appears. It's description is well worth analysing in depth in order to answer your question:
Then saw they how there hove a dusky barge
Dark as a funeral scarf from stem to stern,
Beneath them; and descending they were ware
That all the decks were dense with stately forms
Black-stoled, black-hooded, like a dream--by these
Three Queens with crowns of gold--and from them rose
A cry that shiver'd to the tingling stars...Note the way that the barge is "Dark as a funeral scarf from stem to stern" and populated by dark, black "stately forms." Such mystery and grandeur is of course appropriate for the death of such a hero as King Arthur, and let us also remember that in Arthurian mythology, the dead went to the "island-valley of Avilion," and so it is fitting that a barge collects Arthur and takes his body on to its next destination. The mourning and lamentation that fills the air on the barge is likewise fitting for the passing of such a noble warrior and hero as King Arthur.