Waterlining is slacklining over water. If you're going to slackline, it seems like a pretty good way to do it. Still, it would be hard to walk 728 feet on a slackline for 25 minutes without falling, which is what 24-year-old Mich Kemeter did at Grüner See Alpine Lake in Austria to set a world record in early June. Kemeter fell at least once while practicing, which must have been motivating as the water temperature was 43 degrees Fahrenheit.
Don't let the waterlining fool you, Kemeter has completed more dangerous feats, like a 99-foot untethered walk across a slackline in Yosemite. If that doesn't impress you, a portion of his resume is posted below. The entire thing is a 10-page PDF, so we included just the highlights section: pistol-shooting, BASE jumping, his degree, etc.