Underneath the water, or inside it, is a dark grey flame:

I have seen it over and over, the same sea, the same,
slightly, indifferently swinging above the stones,
icily free above the stones,
above the stones and then the world.
If you should dip your hand in,
you wrist would ache immediately,
your bones would begin to ache and your hand would burn
as if the water were a transmutation of fire
that feeds on stones and burns with a dark gray flame.

-- Elizabeth Bishop, lines from "At the Fishhouses"

At the Fishhouses (poem and commentary by George Szirtes) (link)