A queer thing about those waters: there are no
Birds there, or hardly any.
I did not miss them, I do not remember
Missing them, or thinking it uncanny.
The beach so-called was a blinding splinter of limestone,
A quarry outraged by hulls.
We took pleasure in that: the emptiness, the hardness
Of the light, the silence, and the water's stillness.
But this was the setting for one of our murderous scenes.
This hurt, and goes on hurting:
The venomous soft jelly, the undersides.
We could stand the world if it were hard all over.
I believe that this line is comparing the guilt that the narrator feels from the crime to tentacles of a jellyfish. You try to avoid being stung by the jellyfish’s venomous tentacles similarly to the way the narrator avoids discussing the haunting topic. I also believe that this line may have been referring to the box jellyfish, the most venomous jellyfish in the world. 

Across the Bay