It was first mentioned by name in the episode iBelieve in Bigfoot, where Freddie was watching David Schwimmer (swimmer) underwater. According to Freddie's comments, David supposedly drowned, due to him not moving for a long period of time.
It was later mentioned in iSam's Mom, where Spencer wanted to watch it, but Sam deleted it from his DVR because she wasn't into it and doesn't like the playlist "cluttered" with things she doesn't like.
Carly doesn't like the show because she doesn't think it's fun seeing the D-list celebrities almost drown.
It was first mentioned in Drake and Josh, like many other things in the iCarly universe. In the show, Megan is seen watching it and insulting the celebrities that go underwater, and again in season's two episode 'Drew and Jerry' when Drake was watching Justin Timberlake (who he said to be unconscious), if he had stayed underwater for ten more seconds he would have beaten Queen Latifah.
Also, in the Victorious episode Victori-Yes, Tori and Jade are watching Celebrities Underwater, but Jade wants to change it. Tori doesn't accept right away, but gives in.