Are hummers known for leaks?

Your sunroof has drain tubes that can clog. I would check them to make sure they are draining

no hummer dealership.GM makes them!

I have had the same issue. As Hummertech pointed out, there are four drain tubes in the sunroof area that clog easily. This causes the collecting water in the drain pan to overflow into to roof area and down into the floorboards. The plugs that secure the bottom ends of the tubes have a criss-cross cut septum in them. There are youtube videos showing how to get to these, remove them, drill out the septum, and then put them back in place.