The Lost Continent (1968), produced and directed by Michael Carreras.
I saw at least parts of this decades ago and remembered people walking on the surface of the Sargasso Sea with pontoon shoes and balloons rising from their shoulders to keep them from sinking. That's clever -- would it work?
An odd entry from Hammer Films, source of many oddities. After the unfortunate pop-balladeering title song we have an intriguing first scene: a pan of many hulks stuck in the seaweed, then we hear the service for burial of the dead being read on a large ship and scan an audience of people from different times and places. Finally we see the captain in bright white modern naval uniform reading the service.
The flashback to "how we got here" moves through several phases:
Passengers and crew on a tramp cargo steamer crossing the Atlantic. The captain has a secret and is more than a little shady.
This brings to mind a little micro-genre of the ocean liner as a metaphysical voyage for the passengers, a chance for soap opera theatrics that turns more ominous because the destination is Heaven or Hell. This has been done as a literal plot, as in Between Two Worlds (1944).
Action and survival during the hurricane, wrestling with explosive contraband cargo and mutiny on the lifeboat.
Entry into the Sargasso Sea ship graveyard with its carnivorous seaweed, giant mutated crustaceans and crazed human survivors of an old Spanish galleon.
Notes:
Many familiar faces from other Hammer Films, but Eric Porter as the semi-villain captain is the standout actor. I remember him best from The Forsyte Saga (1967), Antony and Cleopatra (1972) and as a great Professor Moriarty in the Jeremy Brett / Granada Sherlock Holmes series.
One of the Sargasso denizens is a huge-breasted woman whose anatomy compliments her balloons, a striking image. This was singer Dana Gillespie who laughs about it in interviews.
The creature effects are both goofy and actually impressive. Being inventive with no budget.
I don't remember it, but I read that Jules Verne describes the Sargasso Sea in Twenty Thousand Leagues Under the Seas. I think its modern horror-story notoriety begins with the stories of William Hope Hodgson (1877--1918), who also originated the take-care-not-to-be-shipwrecked-on-the-mysterious-mushroom-island plot of Matango (1963).
Prolific author Dennis Wheatley brought Hodgson back into print and the movie is based on his novel Uncharted Seas (1938). A passenger can be seen reading it.
Available on Blu-ray from Shout Factory. A few lower-quality cuts are edited back in to give an extended version. Some of cuts were for mild sex and violence, but mostly for time.
The encyclopedic commentary track is valuable, but could easily have been a booklet.