Not necessary to replace thermo at same time but it is always suggested to do so as well as the gasket.
Here are the removal/installation procedures with torque specs for the 6cylinder 3.7L engine you have listed:
Drain the cooling system.
Remove or disconnect the following:
Negative battery cable
Fan and clutch assembly from the pump
Fan shroud and fan assembly. If you're reusing the fan clutch, keep it upright to avoid silicone fluid loss!
Water pump (8 bolts)
Installation is the reverse of removal. Tighten the bolts, in sequence, to 40 ft. lbs. (54 Nm).
Water pump tightening sequence" 3.7L engine
Saturday, November 22nd, 2008 AT 9:42 AM