It's not so much the fact that they were jet skiing nude, but more of the fact that was intentionally obscene and they were making lewd gestures to other boaters.  This happened on Gull Lake near Brainerd, according to the Star Tribune.  MN conservation officers were on the lake when they were approached by other boaters complaining of the nude men.

Video courtesy of WJON.

Apparently it all started after they got into an argument with a group on a pontoon.  At the time they were wearing swim trunks, but after they left, they returned back bottomless.  According to the complaint, they were also making lewd gestures to the boaters who also had children onboard.

The DNR officers tracked them down with the help of a resort's employee and charged the men with "offensive, abusive, noisy, obscene" disorderly conduct.  The three men charged in the incident were Kody Teslaa, 32, Daniel Klingelhutz, 31, of Waconia, and Ryan Klingelhutz, 32.