I recently messed up the water pump on my 03 600. During some cold weather it must've froze and when I was running it the water pump was leaking from the weep hole. I was able to take it apart yesterday and I discovered that the bolt holding the impeller to the shaft sheared off. This is a left hand thread and 6mm x 1.00 it is impossible to find. Would anyone here have any ideas or maybe an old water pump laying around?

 

I've limited water pump woes in the past but I thought that a water pump shaft had two seals, one water one oil. If you had water weeping from hole then it would of been a seal at minimum if not a new shaft to be safe.
It sounds like your problem is the snapped bolt. Have you managed to get the old one out ?
If you have, then even Honda wouldn't charge the earth for a replacement and at least you would know you were getting the correct pitch and length.
Best of luck and keep us up to date

 

EBay has a ton of used water pumps.