According to the stories I've read on cryptozoology books, some very big eels, giant turtles, mosasaurs, megalodon, giant jellyfish. And we know that giant squids exist, but the biggest one ever found was 'only' 17 metres long. I've read some stories about 40-50 metres long squids. I think there's still a lot of scary stuff hidden in our oceans, but it's probably dying fast due to overfishing and the collapse of marine ecosystems.
The creature in this picture is supposedly a giant octopus that was found on a beach in Florida in 1896, it was called the St. Augustine monster. If I recall correcly it was estimated to have been 30 metres long when it was alive.