Pumice block
Pumice floats on the water surface in the mod (and in reality). If you drop your pumice blocks in water they will float and if the water is moving, the pumice blocks will move in the same direction as the water. Additionally, if you put two pumice blocks next to each other in water, they will break. So be careful if you use pumice as a building block.
So far the pumice blocks are only visible i Creative mode.