The missed opportunity to make this an ecologically-minded story. When this came out (did this one span 1967 and 68?) there had recently been some oil spillages at sea, so the sea fighting back by destroying the rigs seems an obvious route to go down. Instead, while the horror quotient is steadily, and appropriately, high, the titular "fury" is presumably just coincident. A few years later and this would undoubtedly have been more explicitly eco-friendly.
Also, is the link between the foam, seaweed, heartbeats, and Mr. Oak and Mr. Quill ever satisfactorily explained?