This is a gorgeous poem in terms of imagery. I'm sure you know that a metaphor is a type of figurative language in which the author says that one thing is another thing, as opposed to saying it is like another thing (which would be a simile). Metaphors are used to illuminate meaning.
In this poem, there's a stunning metaphorical image created when the speaker says, "I set my broad sole upon silver, / On the skin of the sky, on the moonlight." Let's unpack this. The speaker is not literally setting his foot onto "silver," but he is indicating that the water is reflecting the moonlight, which is making it appear silver. The water becomes "the skin of the sky," a metaphor, which helps the reader picture the silvered lake reflecting the moon in the sky above.
Later, we find, "My fingertips turned into stone / From clutching immovable blackness." Again, clearly the speaker does not mean that his fingertips have literally been petrified. Rather, the indication is that the speaker's fear upon entering the dark water has made his fingers feel numb, cold, and as useless as stone, rather than supple and obedient.
There's also a vivid simile in the comparison of the speaker to "a man who explores his death," which helps us to understand the way he is feeling.