Why was Renton wet after the dream sequence?

The scene has two components, that which is really happening, and that which is happening only in the mind.

In the real world, he has dropped the pill into the toilet, and in the real world, he has put his hands into the toilet to grab it.

In the fantasy world, he completely enters the toilet. We know he can't fully enter the toilet in the real world, but as a high school bully can demonstrate, a human head fits into a toilet bowl. A junkie thrashing about in delirium, one arm fully down the drain, is likely to get himself quite wet. It would be quite plausible for him to make enough of a mess on the floor of the bathroom, that his entire body ends up covered in liquid.

The entire film is a mix of the real and the imagined, with the imaginary often based on the real, but expanded and exaggerated for comic or grotesque effect. I have always taken it as a given that this is a deliberate affectation meant to provide the viewer with a taste of what life is like inside the mind of someone going off and on heroin, unable to determine what is real and what is not.

According to The Wikipedia article for Trainspotting, that scene was copied from a segment of the novel Gravity's Rainbow. Directors often put little homages to favorite works of theirs in throwaway movie scenes.