From 1984 to 1992, there was an official Olympic sport known as solo synchronized swimming. It earns its place on this list perhaps for the name alone. Most any sort of swimming can be considered a respectable sport, but it takes real moxie to call an event that involves just one person "synchronized." (Though solo boosters say the swimmer was synchronized with the music. Likely story.)
Performing complicated acrobatic swimming routines is pretty impressive stuff, and perhaps it can be considered an aquatic ballet. All in all, though, the whole premise was a bit strange. No huge surprise it ended up crossed off the list.