I remember this film being shown every Thanksgiving day for a number of years when I was living in Washington state.
Eric Porter stars as a ship captain who is hauling dangerous cargo and a few passengers. The ship hits a typhoon and begins taking on water. The cargo is highly explosive and the passengers unload it as quickly as possible. They man lifeboats and are taken by the current to a strange land where the seaweed is hungry for blood and weird monsters live.
They also run into a bizarre race of people living on a ship in an ocean graveyard! these people live like medieval times and are ruled by a boy king. The monsters resemble mutated sea life and are quite a sight to behold.
This is one of the most bizarre Hammer films ever made, but it is very entertaining and being one of a kind doesn't hurt either. The rest of the cast includes Michael Ripper, Suzanna Leigh, Benito Carruthers, Hildegard Knef and Tony Beckley. Long out of print, this is one worth finding.