Sea monster accounts are found in virtually all cultures that have contact with the sea. For example, Avienus relates of Carthaginian explorer Himilco's voyage "...there monsters of the deep, and beasts swim amid the slow and sluggishly crawling ships." (lines 117-29 of Ora Maritima).
Charybdis- Greek myth: a huge female sea monster. She was the daughter of Poseidon and Gaia. She was extremely fat and to satisfy her hunger she stole the cattle of Heracles. Enraged, zeus cursed her, and turned her into a sea monster. She now sucks in massive amounts of water three times a day creating huge whirlpools.
The Fomorians from Irish mythology are steeped in mystery. It is unknown where they came from, or the length of time they had knowledge of Ireland, and what actually happened to them. Lady Gregory describes one Fomorian habitat as a glass tower in the sea. Many people describe them as sea fairies or sea monsters. My best guess is that they were Nephilim!