A ship captain is hauling illegal explosives on his boat, along with a suspicious group of passengers, all of whom have sordid histories. During a hurricane the ship starts taking on water--and if the chemicals of the explosives come in contact with water they will explode. The crew abandons ship first, then the captain and the rest of them. However, they end up being caught in a weird morass of floating seaweed and run back into their ship, which has weathered the storm.
The seaweed is alive and hungry, with these tentacles. The ship can't get out of the vegetation so it floats along until it comes to a much much bigger cluster which contains the remains of dozens if not hundreds of ships it has caught. They are not only attacked by weird plant monsters that reach into the ship, they encounter a group of Spaniards who are descended from early explorers and want their food and supplies. They fight and a lot of them are killed. There's also giant crab and scorpions on one of the rock outcrops they have to survive.
This is a good, "Lost on an Island" movie, only there's no island. Based on the novel UNCHARTED SEAS by Dennis Wheately