Yep, as the guys have said, there is no spring of water bubbling up under the back of your front seat. It generally comes from up front, and your rear footwells are the lowest part of the inner car tub. Often the water travels under the insulation that is added on the top of the floor pan/tub.
Windshield seals, front and back are notorious for leaking. I had a flooded rear footwell on the wagon and the
Squash after this last rain. I suspect windshield, as I've eliminated all the other usual suspects.
And, on your temp issue, that is definitely too warm. Assuming you have verified correct water/coolant level, thermostat would be your next best bet. They actually do often go out it seems [I've done 2], and they may be one of the easiest cars ever to change them on.
Also, notice how it comes out, and don't get it in backwards. Matter of fact, you should make sure it was positioned correctly when you remove it, as a previous incorrect install can also cause it to run hot. I will be back with a link.....
**as promised**
http://dieselgiant.com/thermostatreplacementandmodif.htm