There are several specific indications that the sea is at high tide. The opening words of the poem state:
The sea is calm tonight.The tide is full...
Listen! you hear the grating roar
Of pebbles which the waves draw back, and fling,
At their return, up the high strand,
The Sea of Faith
Was once, too, at the full, and round earth’s shore
Lay like the folds of a bright girdle furled.