Guys, don't be afraid of casting! It's a fascinating and very useful skill. Sure, you don't want to let your kids cast an engine block, but if you stick with aluminum, the temps are low (that's relative, of course) and with modest precautions you'll be fine.
I've never had the slightest problem with moisture. Of course, if you INTRODUCE moisture, you're asking for trouble, but by the time the aluminum melts, any existing water is driven off.
The real problem with water is not eruption, it is bubbles in your casting. Atmospheric moisture reacts with molten aluminum, causing minute Hydrogen bubbles to form in the aluminum when it is poured. As the metal chills, the H2 bubbles come out of solution and are frozen in place after total solidification. This can be corrected before you pour with a simple "degas" procedure which uses dry nitrogen, argon, or commercial pellets.
Here is a link to my little foundry page:
http://www.5bears.com/foundry.htm
Note: DO NOT pour over concrete like I did here, that CAN cause problems with a spill. Pour over dirt or sand.