Oddly enough... I found myself swimming in lava the other day.
For the record, in deep lava you can swim down, just like water. The Demon Queens lava pool isn't that deep though, perhaps 6-8 feet total. The bottom was rather featureless. I was a bit disappointed, it would have been a great place for odd easter eggs. Or perhaps a collectible spot that is nearly impossible to use.
Mind you.. I was dead. Its much safer that way. (We had someone die and get stuck in a wall in the lava in the raid. So post-completion I, and a few others, tried resurrecting. We'd get out of range errors from the top, and out of range from the portal safe spot. So we'd jump off and try while floating down..... Once mid float I did get in range... and got "blocked" as my error. On one try I was able to successfully do the lavahop/heal trick back to the portal... on the second try I died on the "coast" of the portal spot. Couldn't do it though, no raid loot for him.... he was stuck good, even his ghost couldn't move.)