Can you really swim in lava?

No. There are 3 reasons.

1. Lava is extremely hot, and falling in lava would result in certain death.
2. Lava is dense, about the same density as rock, so you would not sink in far enough to swim.
3. Lava is viscous, so ignoring the first two reasons, it would be like trying to swim in molasses with the most fluid lavas.