I am using the definition of consonance that says that it is when you use the same consonant sound in a bunch of words (more or less in a row). The sound does not have to be at the start of the word like it does in alliteration.
There are a number of examples of this in the poem. Here are a few:
- In the first couple of lines of the third stanza, we have this, was, reason, this, sea. All of these have "s" sounds.
- Again, in the fourth stanza, you have was, reason, as, this, sea. Again, lots of "s" sounds.