It may be the water pump if you are sure the system is not plugged up. Is the coolant clean and free of slugde and rust?
If it is the the waterpump, the below is the replacement. You may be able to skip the alternator part, but it is much easier this way.
Before starting, drain as much coolant out as you can using the drain on the drivers side of the radiator (on the bottom .. just remove the splash gaurds first. Makes accessing it easier and keeps it from going all over while draining).
1.Using a 19mm socket on a breaker bar on the fixed nut for the tensioner, pull it towards the front of the car to release tension on the belt. Don't pull hard past the stop, or you can break off that nut.
2. Remove belt
3. Remove the water pump pulley bolts. You can get the special tool .. or like most of us, use a large screwdriver between one bolt and the center hub to hold it while you loosen each bolt. After all are loose, remove them with your hand
4. Jack up car, remove passenger side wheel and the small plastic cover that is in front of the crank pulley (two screws and a push pin).
5. From beneath, push the water pump pulley out of your way and you will see the bottom bolt for the alternator. Remove it with a 12mm socket
6. Remove the 14mm bolt and wiring connections (one will take a 10mm wrench) from the alternator and remove it.
7. Place drain pain under the water pump area to catch coolant that will drain out. With a 10mm socket, remove the bolts from the water pump and pull the pump free.
8. Remove pump from the top and clean the gasket surface well.
9. Install new pump and gasket.
10. Install Alternator
11. Install water pump pulley
And then be glad you don't have to replace the tensioner .... two easy bolts ... except the top one requires lifting the engine 6 inches to remove
I hear they may have changed that on the 07's though.