: suggestions for securing hydrophone coax to the hull as a stop before cable
: runs into a Pelican box?

There are these contraptions made of stainless wire that are used for securing loops of cable. I don't know if they come in small sizes, and I don't even know what the polite name is. Ask an electrician about "horse's c**ks".

But failing that, I would tie a prussik around the coax using some nice limp braided cord, then clip that to your deck rigging with a carabiner.