Ok , I ckecked and the water pump is gear driven and you can't see it. Well not enough to inspect any way. Do you have a cooling system pressure tester? I use that to determine leaks, head gaskets and such. You can put it on in place of the cap & read pressure build up & pressure drop. Was the water pump changed to correct this issue or did it appear after? Have you owned the machine for a while?
If all the basics have been checked: fan isn't damaged & is turning easy, coolant is the proper type and level, radiator is clean and free of debris, grille screens are clean, oil level is correct, load isn't too large for machine (6ft cutter isn't), coolant guage is functioning properly, coolant system is clean, thermostat is functioning properly.
That's where I would start. If all those check out, it may be time to check in more depth.
-Was the timing changed when the water pump was installed? The injection pump is right behind the water pump. If the guy didn't know what he was doing it could have been altered.
-Head gasket issues? Was the machine run hot enough to fail a component?
At 400 hours this tractor is still just getting broken in. I'm surprised to hear of a water pump replacement.