There are so many word referring to the sea in this poem. Let's look verse by verse.
The first is the word "sea" itself in the first line. In the second line, we have the word "tide," which must refer to the sea, since, to the best of my knowledge, lakes do not have tides. In the third line, we have the word "strait" and the word "coast." A strait is a narrow passage that connects two large bodies of water, and there is an inference here that these are seas, rather than lakes, because of the rest of the poem. We cannot really have a coast unless we have land on a body of water, so this word, too, really involves the sea. Moving along in the poem, in line 5 we have the word "bay," which we need a sea to have, and in lines 7 and 8 there is:
...the long line of spray
Where the sea meets the moon-blanched land
The word spray refers to the water spraying when the waves hit the beach, and then there is the word "sea" again. Later, in lines 9 and 10, there is:
...the grating roar
Of pebbles which the waves draw back
Have you ever heard this roar on a beach? While waves could refer to a lake, we know we are at the sea in this poem.
In the second verse, in line 16, there is a reference to the Aegean, which is a sea. In line 19, the poet uses the words "ebb" and "flow." He is using these words to describe human emotions, but these are words about the movement of the sea. And in line 22, we see the word "sea."
In the third verse, there is only one reference to the sea, in line 23. That links together the literal sea that is the setting of the poem and the poem's larger ideas.