Today the leviathan is not considered to be one of the most popular sea monsters but is considered to be one of the most believable due to its mention on the bible.It is mentioned many times and has a real descriptive mention in Job chapter 41 and is said to be a creature of immense strength and size. Many scholars have read about this monster and believe it to be a crocodile but others think differently due to the fact that crocodiles don't swim in deep water and don't reach the size described by the bible. However there have been reports of crocodile resembling creatures by submarines and they have been said to reach the size if 15 to 18 meters. However there have been no pictures or real evidence of this.

http://www.answersingenesis.org/creation/v19/i4/seamonsters.asp