Please summarize the last two lines of the poem "The Survivors" by R.S. Thomas.What is the meaning of the line "A vast disc under a dome" and why...

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:

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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.