Possibility (a) from Golf with Rain Problem:
► Moisture completes a circuit somewhere, causing the battery to drain.
If this is the case:
1. It should be possible to jump start the car and/or recharge the battery, and then get it going again once it's dry. And indeed, it is possible to jump start the car.
2. Narrowing down where the circuit gets completed would be the project. How can this be determined?
► The battery could be damaged in some way that it doesn't hold a charge long enough to start the car.
If this is the case, see Ward's suggestions on Diagnose Golf Rain Problem