1999 Mazda Millenia Water/Coolant leaking, engine running h

Yesterday, my daughter's mazda was leaking water when she was driving and engine was hot. We immeidately took it to a mechanic and they did a compression test and said it was either the head gasket or maybe a cracked head, they couldn't tell.

We had a used engine (35,000 miles) put into this car in Mar. '07. I am questioning the findings of the mechanic. Could this be a water pump? What would cause a gasket or head to go bad w/this little miles on the engine? (Also, when the used engine was installed in 3/'07, a water pump was also replaced.