"On The Sea" is a 1817 poem by John Keats.

The poem is short, so here is the whole thing:

ON THE SEA
by John Keats

It keeps eternal whisperings around
Desolate shores, and with its mighty swell
Gluts twice ten thousand Caverns, till the spell
Of Hecate leaves them their old shadowy sound.

Often 'tis in such gentle temper found,
That scarcely will the very smallest shell
Be moved for days from where it sometime fell.
When last the winds of Heaven were unbound.

Oh, ye! who have your eyeballs vexed and tired,     Feast them upon the wideness of the Sea;
Oh ye! whose ears are dinned with uproar rude,
Or fed too much with cloying melody---
Sit ye near some old Cavern's Mouth and brood,
Until ye start, as if the sea nymphs quired!
(Keats, "On The Sea," eecs,harvard.edu)

The major theme is that of the eternal ocean, which has lasted for millennia and will continue into the future. The sea "gluts twice ten thousand caverns" and covers an immense area compared to the land we humans live on. Despite the great amount of sea life, it appears to us to be flat and featureless: "the wideness of the Sea" is more than we can comprehend. Some myths and legends -- and legendary figures -- are strongly associated with the ocean; Hecate, a Greco-Roman goddess, is associated here with the tides, "leave[ing] them their old shadowy sound." The overall sense is of an immense and unmeasurable place beyond human ability to tame or fully understand.