Contrary to what you might have seen in movies, if you jumped into lava you would not sink. Lava is nothing like water. Lava is a term used for molten rock on the surface of the earth and is incredibly dense, over 3 times the density of water.
Water has a density of 1000 kg/m3 and Lava has a density of 3100 kg/m3
Humans have a density of around 1010 kg/m3 which is very similar to water and this is the reason that we can float and not sink in water. However lava has a density much higher than our own so we would not sink or float, we would be stuck on the surface.