It needs to be long also. Most of us have picked up a bicycle spanner in our travels. I can't remember the link for when I got mine (may be on the archives, but it was a bike shop). There are two schools- whack the spanner with the belt on to loosen it, or, put the belt aside, and put a rubber strap wrench around the water pump pulley, and do a push/pull. I vote for the latter.

If you get skunked on finding one, I have a spare to sell you, but you'll have to beg and grovel. Perhaps you have trade bait?