Many years ago, I read a rather thin sci-fi book about some scientists finding mutated sea life caused by pollution coming from cylinders dumped at sea by some bad guys. This included a couple of mutated giant squid like creatures who had eggs in an underwater cave. The scientists ended up blowing up the cave and killing one squid to stop the eggs hatching, and the other squid washed up on the beach. This monster on the beach ended up killing some of the bad guys as they were running along the beach.
The cover had a picture of one of the giant squid like creatures with its tentacles flowing around the place, and for years I had the thought it was called "Deep Six" but I have had no luck whatsoever in finding the book.