So I'm buying some plywood from the hardware store...anyone know how I can treat it so it'll be moisture-proof but at the same time non-toxic to the plants?
Check this out to get you started
Repurposing a plastic kids swimming pool for a raised bed would not be my first choice for a well-considered garden design :-)
If you are using plywood (again, not my first choice for raised bed construction), make sure you get marine quality or exterior grade plywood - the regular, cheap stuff will not hold up regardless of treatment. And you should be aware that most plywood has glues and other chemicals implanted that would not be considered "organic" or non-toxic with regards to growing veggie crops. There are some 'green' plywood products but they will be pricey.
There are low/no VOC or environmentally friendly wood preservatives on the market and most home improvement centers will carry at least one or two brands. However, they are not necessarily formulated with food safety in mind :-)