When spawning a custom world with "lava oceans" checked, and "biome size" ramped up to 8, my player has spawned underneath a lava or water ocean.

Whether this is a problem with the spawn mechanics of 15w46a, or a problem with the "lava oceans" button, I don't know.

I have repeatedly re-created worlds to see if it was just un-lucky circumstance, but the issue is persistent.