Steam is the only substance so far that floats upwards. Steam is generated when either Water or Poison Water touches Hot Rocks or when spewed from a pipe. Steam can be converted back into water by Icicles.
Steam can be contained in Balloons, and when the Steam balloon is filled, the balloon will float upwards. Balloons filled with Steam have a wispy, smoky design on them. Steam can float through Algae.
In Where's my Perry, steam lasers turn water to steam. When Steam interacts with Mud, the mud stays in its wettest form.