This is a rather famous scene from episode 37 of Death Note. Parodies of the scene have become fairly common, including a Know Your Meme page. For an example, this video has clips from the scene set to "Native Faith", the extra boss theme from Touhou 10. Misheard or altogether wrong lines from this scene are also common, as is the case in this particular GIF. Here's the original video; as a warning, it contains major spoilers.
As for what's going on in the scene, again, it's very spoilery, but here's a description. It does not involve swimming.
Yagami Light has just been found out as Kira, the serial murderer, who possesses a Death Note, a notebook with which he can kill anyone by writing their name in the book. Matsuda, a police officer, had a friendly relationship with Light (as well as looking up to Light's father the police chief), so discovering that Light was the criminal they were searching for all-along, and was responsible for his own father's death (as well as many others), was fairly shocking. Light, on the other hand, at this point has succumbed completely to his insane delusions of grandeur, and as he has no more tricks up his sleeve, he attempts to persuade the officers who have caught him to join forces with him, but comes off just looking even more insane. As a result of all of this, the scene is rather dramatic.