There was actually some interesting things going on here, though taken in their separate parts they sound somewhat silly. There was a gigantic seaweed monster or something, w/two heads -- at least I think they were heads; could've been eyeballs -- that should have been more ridiculous than it was; the overall effect was rather graceful AND menacing. Loved how amidst everything else that was going on when the guys on the Seaview finally saw it they didn't bat an eye. There was a DC comic book being published around this time called "The Sea Devils" about a team of stalwart scuba divers who were forever encountering giant undersea monsters; wonder if it was influenced by this show or perhaps vice versa.
Lot of balls in the air in this one: An arrogant, fame-seeking admiral with daddy issues; the testing of a revolutionary pressurizing gas; the simmering hatred between the admiral and the gas's inventor; territorial tension between Nelson and the visiting admiral; a near-death scrape for Cap'n Crane; and even a little bit of conflict between Nelson and Crane. Throw in the monster and you've got a fast-moving, compelling 50 minutes of bizzaro drama that kept me interested. Also some nice overhead camera shots when they were desceding in their little 4-man, vertical submersible thingy. They managed to create some cool effects out of a meager budget with this one.
Some familiar faces among the guest-stars, including J.D. Cannon who would go on to be a foil for Dennis Weaver on "McCloud", and Alvy Moore, who would become beffudled county agent Hank Kimball on "Green Acres."

If you liked the show, you will like this episode. Significant action, w/o too much drama. Sea creature typical of era.