You have certainly picked a poem with a number of separate examples of personification in it. Let us remind ourselves of the definition of personfication. Personification is a figure of speech which gives an inanimate object human characteristics and is a form of comparison. When we bear this in mind, we see that this excellent poem contains personification which helps create the dark, sombre mood that dominates its lines. Let us consider the first example:
A little light is filtering from the water flowers.
Their leaves do not wish us to hurry:
They are round and flat and full of dark advice.
Note here how the water flowers are given human characteristics. They do not "wish" the speaker and her companion to hurry and are "full of dark advice." The "dark" advice gives them a sinister perspective. The third stanza contains the second example of personification, with the "snag" that lifts a "valedictory, pale hand." Notice how this goes with the "spirit of blackness" and the "cold worlds" to emphasise the dark, depressing tone. The snag is here given a hand, comparing it to a sad farewell from someone.
There is one more example of personification in this poem, but having identified the other two, I will leave you to find it for yourself. Hopefully now that I have identified two examples for you this should be an easy task. Good luck!