Trifox

Wilson can be found in World 1, Level 2.
Just go to the left side of the beach and pick him up behind some crates.

You can throw him in the ocean (quickest way) or carry him to the end of the level and throw him in the pit (more dramatic way and thus, more fun ^^)

Use the skips/chapters in the video below:

Can confirm this achievement did not pop for me by throwing Wilson in the ocean in the beginning of the level. He just kept coming right back up to the shore from the current. It did however work when I threw him into the pit towards the end of the level. Make sure to throw him in before proceeding to the "spider like octopus" boss or you will have to repeat the level again. :)