It could be the oil sender, but only IF the sender were located in that general area. I've included a few pics I just snapped today under the hood. In the first image, is where the oil sender is located... on a 94-98. It's the round grey thing with the black plug on it just to the right of the water pump. If I'm not mistaken, the heads and intake are different on 99-04 V6, but the block and timing cover are the same. The sender is attached to a tube that comes from the timing cover. If oil were leaking from that, it would not make its way onto the water pump valley area.
The next image is the same side of pump that's in your picture. It's more difficult to see because so much is in the way. As you can see, I also have nasty leakage problems. Only I believe the wetness on the block is coolant from the head, and the stuff on the left side of the pump is oil, probably from the upper side of the timing cover. The cover has been leaking small amounts for years.
The last image is on the other side of the pump, just next to the water neck. All that mess is coolant coming from the head, and where the head meets the lower intake and block. I just cleaned all that mess off from both sides 2 days ago. There wasn't a drop of anything on it when I was done. Now look at it. Seems to be getting worse every month. That small dry looking spot in the middle of all the wetness is where I just sprayed ... seems futile to clean it now.
Now that you sen these, you can get a better idea of what may be leaking. I suspect either your timing cover (if its oil) or the cylinder head on that side if its coolant. Forget where the water neck is on 99-04 models. If it's in the same spot, then that could also be leaking. Or the pump. Best way to find out, spray everything down with concentrated simple green, let sit 5 minutes, then hose of. Drive for a halof hour then check for leaks.
