|(Note: This involves entities, entity populations, *and* worldgen, so I wasn't sure how to categorize it.)
So, I noticed that humans had [BIOME_SUPPORT:ANY_OCEAN:12]. This would imply that they spread out to shorelines very often, but I don't generally see this happening.
As an experiment, I gave humans ANY_OCEAN as their only START_BIOME.
The results were completely screwy. They were building hamlets and towns in the middle of the open sea, and spreading seemingly like normal. Starting an adventurer, whether a dwarf or human, attempted to place me in one of these hamlets, with the result of myself starting off in a cavern, as a nameless crundle, surrounded by "peasants" who were also nameless, and also crundles. The description screens of my adventurer and my fellow crundles corroborated the fact that, yes, we were crundles.
1) BIOME_SUPPORT for oceans probably doesn't actually do anything, whereas entities attempting to *start* their civilizations in oceans leads to extremely funky behavior. I assume the ideal behavior here would be for humans to prefer spreading out near the ocean, along the shore of that biome but not in the water, like they do for rivers, and for some kind of sanity-check to prevent building structures in the open sea (except in some future case where this might be appropriate).
2) The game acts incredibly freaking fishy (no pun intended) when it attempts to place an adventurer at a place where expected features/populations are not present, or perhaps simply when placing an adventurer underground. Maybe it's drawing creatures from a different list, or something, because the underground is a different biome/region? I don't know.
I assume the above happens with lakes as well, but I haven't tested it.