Hydrogen is less dense than air, yes. That's not hydrogen being used in the video, though. Well, maybe if the entire auditorium was filled with toxic concentrations of hydrogen, and the tank was full of air, but then I think the predominant audience activity would be panic, rather than applause.

Great, now I'm obsessed with figuring out WTF that stuff was. Assuming the whole stunt wasn't performed in a pressure chamber, and calculating via the SWAG method that the boat massed about 50 g and displaced about 500 ml of the denser gas, it means that, in order for the boat to behave as it did, the denser gas had to be over 80 times as dense as the surrounding atmosphere.

The densest gas I know of, radon, is only 9 times as dense as air (and would NOT be safe to screw around with bare-handed). That gas would have to have a molecular weight of over 2000 g/mol.

Barring wires, magnets, pressure chambers, etc. this would be a showcase of some of the most amazing materials engineering I've ever seen. I'd be more inclined to believe they used a liquid with the same index of refraction as air, than such a gas.

Which actually makes sense, since the video is titled "invisible water," no matter what the caption says.

I thought one of the denser noble gasses would do the trick, but sounds like you already thought of that.

... Except there's a whole pile of phase transition research that indicate that no solid or liquid can have anything close to the index of refraction of any gas, at any survivable temperature and pressure. Light of any wavelength is simply slowed down too much by the proximity of the electron shells in any solid or liquid.

Barring any sudden announcement of a breakthrough that makes the technology columns of newspapers worldwide, I'm going to have to figure it's just a well-performed magic trick.