One more piece of new information. A number of sources have said that Boxsters have 4 drains, 2 in the front and two in the back -- one of which is pictured in gsmiller's post.
I found another pair of drains for the real drip-rails that should most likely also be checked to make sure that they are cleared. Here's a picture of where you can find the drain on the right rear of the car.
It's right up front, next to the central hinge for the roof. For reference, in the picture I have the roof in the service position.