Seforno: It refers to the sin of immorality, which tends to increase and from which a person is never satiated - as the Gemara states in Sukah 52b: 'There is a small limb that a man has - If he satisfies it, it is hungry for more'.

Oznayim la'Torah #1: Because Hashem sent the angels - the Nefilim - down to earth to teach the people how it is possible to serve Hashem even on the physical earth, yet they fell from the level of 'heaven' down to the level of 'earth', when they began to follow the example of the Resha'im and even to exceed them in their wickedness. ¹

Oznayim la'Torah #2: Because Hashem formed the earth in order to establish a system of justice ² - as opposed to the sea, where the big fish devours the small fish and which is devoid of any form of justice. Consequently, when He saw that the land was no better than the sea - that theft was rampant, and the justice for which He had created the land was non-existant, He decided to revert the land to water as it was originally.

Seforno: He saw that man had been evil up until now, and that all his thoughts - every hour of the day - were focused on sinning further, so that there was no chance that he would pay any attention to anyone who came to rebuke him.

Peninim mi'Shulchan ha'G'ra: One who is defeated in war becomes weak. Yet, when one defeats his Yetzer ha'Ra, it becomes stronger - "the greater the person, the greater his Yetzer" (Sukah 52a).