With genetic engineering thus presented we are delivered a horde of nasty killer abominations, my favorite is the killer kelp which infects it's victims. The poorly designed Siren 2 is able to far exceed that record with a hull crush depth of 35,000 feet. At 27,000 feet they find the naturally air-filled cave where an SOS signal is coming from, investigating the cave thins out the cast nicely as the mutant beasts have a feast. Robbins nearly achieves his goal of destroying the facility and leaving no witnesses but quick thinking on Wick's part saves our heroes. Well, two of them, Capt Phillips stays behind to set off the Siren's self destruct after being infected by the kelp.
Nobody wants to share a cabin with the seasick guy.
Top secret submarines navigate by sight and normally pass within 15 feet of obstacles. A human being can dive in a dry suit at 20,000 feet. There are naturally air-filled caverns at 27,000 feet underwater. Stay away from perfectly shaped tunnels dug into rock. When on a sub don't drink the water.