A speaker carries a thesaurus down to the sea, holding it the way you would hold a small child. That tender image is a little funny, since a thesaurus is a book of words, not a living thing, and it hints that the speaker trusts language to help make sense of what's ahead. The setting is a port, where boats gather and, as the poem puts it, "confess together / and fall out." The boats seem to be speaking, perhaps through creaking hulls, clinking rigging, or the sounds of many harbor languages overlapping.
But what they say is "a bland discourse / in three dozen tongues." The sea's many voices are plentiful and yet oddly flat. The speaker arrives with a stock of synonyms, only to find a babble that no list of words can sort out or sharpen.
The mood is quiet, wry, and a bit wistful. There's affection in the child image, and gentle humor in the mismatch between book and harbor, but also a sense that language, however well stocked, can't quite capture the sea.