Hi. The water pump on your 3.5 runs off the timing belt inside the timing cover. It takes a good bit of disassembly to get to the pump. Once you are there you have to remove the timing belt because the pump acts as an idler for the belt. If you replace the pump replace the belt at the same time.
This isn't really a job for a basic do-it-yourselfer. You have to remove the core support, fans, belts and timing cover. You'll need a specail puller to remove the crank damper, along with an assortment of metric hand tools. If you don't get the timing belt put back on right you can do some engine damage. The valves could hit the pistons if not done correctly. This job takes about 3-4 hours for a trained tech with all the right tools.