The Lost Continent (1968)

On their way to South America, the passengers and crew of an old freighter face many challenges. The captain has taken on illegal cargo--a dangerous consignment of phosphorous that will explode if it comes into contact with water--something that worries the crew but less so for the passengers who all seem to have good reasons to continue their trip. When the ship takes on water some of the crew mutiny and eventually abandon ship. For those who stay aboard, they soon find themselves trapped in a huge bed of living seaweed and find a several centuries old Spanish galleon that has been trapped for all of that time - now manned by the descendants of the original crew. They need to help them if they are to survive.