A Siren isn't really a sea monster but sailors have told tales of these beautiful women that sing to you and seduce you into their trap of bloodlust. They are said to eat the sailors alive or drowned them deep in the ocean.
Putting the Size of a Whale in Perspective! Roughly the size of a basketball court, the blue whale is the largest mammal known to have ever existed. Little did this kite surfer know the creature swimming just beneath him!
Cthulhu is a malevolent entity who hibernates somewhere beneath the ocean in the South Pacific. The imprisoned Cthulhu is the source of constant anxiety for mankind at a subconscious level, and also the subject of worship by a number of human religions.
megalodon was a prehistoric shark that conservative estimates place at around 16 metres ft) long. For scale, the biggest example of the largest living fish species today -- the whale shark -- was metres ft).