The subject of R.S. Thomas' poem called, "The Survivors," may provide us with an answer to the question one has at the end of the poem: what happens to the stranded people in the boat?—or it may not answer the question at all: the "survivors" may be those who live through one disaster (the sinking of a ship, perhaps) only to face another disaster—being lost at sea.
From the first two lines, we know that at least one person survives: the "he" that "told me about it often." The poem progresses by telling of the days lost at sea, while the poor occupants of the small boat try to survive, and the certainty of death seems to loom closer each day, though they don't speak of it:
They began to think about death,
Each man to himself, feeding it
On what the rest could not conceal.
The definition of "a vast disc under a dome" comes from the line directly in front of it. (Watching the punctuation at the end of a line, or the lack of it, can often shed light on a poem's meaning when you can isolate complete thoughts.) The line before it:
The sea was as empty as the sky...
is the context in which we should look at the first line you mention. In other words, the thought reads (but does not end with):
The sea and the sky were empty: the sea, a wide disc (round object), is under a covering or a dome-shaped roof...
The sky is often referred to a dome over the earth. So the sea is the "vast disc" under the sky, but also vast is the sea—I believe the comma after "vastness" means that the poet is still referring to the sea when he states that it is "perilously blue."
...and the sea is perilously blue.
It is described as blue, simply for the color of the water (I believe), but it is "perilous" because it seems never-ending. There is no rescuing boat on the horizon, no sighting of land. The longer the survivors are on that vast sea, the more likely it is that they will die—that help will not come in time.
We are left to wonder as to the poem's end. In order for the story to be told, someone must have lived to tell it; and we understand that one person survived (as mentioned earlier). As to the fate of the other occupants, we have no way of knowing.
The poem talks about surving christmas