Yes, it was the mechanic, but all the symptoms seem right from what I've read here. He showed me the old one and the plastic had broken into 4 pieces. Based on the information I gathered here, and from calling the dealer and pep boys, I told him he was over-charging me. The owner redid the estimate and charged me $350 in the end to do everything.
$170 labor, $155 parts (pump, thermostat, coolant, & belt) +tax.
I got my car back today. Seems to be OK so far. A while ago someone on this forum was collecting information (to petition Ford or class action or something) from people who've had waterpump failure, especially those who had major damage as a result of it. Does anyone know if anything ever come of that?