Forget everything you've seen in movies, cartoons and video games Cauldrons of an angry volcano, you will not sink into the fiery pool like a drowning swimmer. Given the reality of the situation, however unfortunate it may seem, it would be a preferable alternative.
Lava, as it is popularly described, actually has a much higher density than water and the human body. In addition, it is highly viscous and has more to do with peanut butter than water. As such, you would not sink as much as you would lay on it like an egg in a pan.