There is no water in this glass tank, yet the aluminum foil boat floats. On top of that, the substance that the boat floats on can be scooped up with a regular glass and be poured
onto the boat to make it sink. It's not a trickery or any sort, it's just physics being in play.
The substance that's in the glass tank and is being poured is Sulfur Hexafluoride. The density
of this substance is what enables it act this way. Sulfur hexafluoride is almost five times as dense as air, meaning that more molecules of this specific gas inhabit a given space than molecules of other gases.
The sulfur hexafluoride is also a heavier gas than air. The combination of both these attributes enable it to sink to the bottom of the glass tank here and stay there, effectively making it a pretty dense fluid. This allows the gas to be scooped up and poured by normal means, just like a liquid.
This enables the aluminum boat to float on the surface of the fluid, through what is called displacement
meaning the boat displaces the fluid in proportion to its weight so it can float. So if the boat weighs 100 grams, it would displace 100 grams of the sulfur hexafluoride to float on its surface.
In order for the boat to keep afloat by displacing its weight before sinking to a particular point, the shape of boat is made to be buoyant
. This enables the boat to keep afloat at a higher level when floating on the fluid. Also, this would only work with a boat made of light material like tin foil, because the fluid at work here is still only a gas, however dense, and isn't a liquid.
More detailed explanation of what's going on here: http://science.howstuffworks.com/environmental/earth/oceanography/invisible-water1.htm
Facts and other used for sulfur hexafluoride: http://chemistry.about.com/od/demonstrationsexperiments/a/sulfurfluoride.htm
Short YouTube video demonstrating what's happening, and source of the gif: Invisible Water Floating Foil Boat- Actually a Dense Gas... Called SULFUR HEXAFLUORIDE