LiPo batteries explode if you short them. Doesn't even need to be a very long short, or a very big one. They are incredibly sensitive to fast discharges. Water easily conducts well enough to cause problems, especially if it messes up another power conduit. It isn't guaranteed to catch fire, but the risk is certainly there, especially in such a thin form factor - you don't have much space in there for containment or fancy routing to reduce the chances of a short.
Again, it's not guaranteed, but Airs are rather more at risk than other laptops because of their construction.
The battery is ok for direct exposure to water as it's well sealed but as you say the power output from them in short circuit is immense which results in fires if you can't isolate the supply. Water + high current = interesting :)
use http://www.macupdate.com/app/mac/23049/smcfancontrol - when the thing gets hot, just increase the fan speed (to 5700 rpm or whatever you machine's max is) for a minute or two, and then everything gets back to normal.
Again, it's not guaranteed, but Airs are rather more at risk than other laptops because of their construction.