I've noticed today after a very heavy rainstorm that my back seats are completely soaked in water, with some accumulation on the floor under the seat as well. Nothing else is wet, the trunk is dry, floor carpet is dry, same with the seat backs and the interior panels.

Could it be a bad roof seal? What would be a good fix for this?

 

Unlikely, check the sunroof drain tubes, they're more than likely clogged allowing water to run down the rear pillars and into the back seat, you wont see it drip directly from the sunroof. The sun roof seal is not water tight.