How do I troubleshoot water pump? With tension, serpentine runs off pulley. on 1997 BMW 740iL

Wife got stranded when she heard loud rattling noise and noticed coolant from under front of engine. Then we saw that serpentine belt runs off of water pump-fan pulley. When I put belt back on and re-tension, it runs 1-2 way off as soon as I start the engine. Possible for pulley to come loose and tilt down under pressure?

take to a shop an have it pressure tested to find the leak
1 more answer
Replace Belt an Pully