Could be several problems, and possibly the cause for the broken belt in the first place.
Make sure the water pump pulley isn't installed backwards, it's dished as I remember, and if someone put it on wrong, the pulleys will be off. This will eat a v-belt pretty fast, they don't tolerate much sidways flex.
Water pump may be pulling out of the bearings, in which case you will have overheating problems pretty soon, too.
Check with the crank pulley, too -- if it lines up with the alternator, for sure the water pump is off. If the alternator pulley is off, check to make sure no one has replaced the pulley there -- that is a Motorola/Bosch unit, and several American applications use a similar one, someone may have swapped a different one in.
If you need a core, I have one (doesn't work, probably has bad diodes).
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!