I've turned on pond mode (and set gravity on, since the bots weren't sinking or anything),
Screenshot of General tab of match setup:
http://screencast.com/t/SBnncM66BNScreenshot of Physics and Costs tab:
http://screencast.com/t/1jbDtxjOI set gravity on and changed the fluid to thick, since pond mode apparently didn't. It didn't change the toroidal/wrapping, or corpse settings either, but I stopped changing things once I saw buoyancy wasn't working.
Buoyancy is definitely either broken or off: I've got bots in the sim who are setting setboy to 2000 (every cycle), and rdboy is staying at 0.
From the console of one of those bots:
printmem .setboy
314-> 2000
printmem .rdboy
315-> 0
robot genes execution: - =not executed
1 executed
2 -
3 -
4 -
5 -
printmem .rdboy
315-> 0