mechanic tells me water pump went out..causing water in engine. Symptoms: truck was running but low power, no acceleration
Best guess based on your explanation: First the water pump quit/died, then engine overheated as a result and it remained running overly-hot for sometime thereafter, which (likely) led to head gasket failure, which resulted with coolant in the oil and the performance symptoms.
So yes, with the right unfortunate series of events, a water pump failure can result in coolant in the oil. But I believe it requires a key gasket failure somewhere in between. For what it's worth a failed water pump alone will not result in immediate performance symptoms, so as @JPhi1618 is suggesting, the symptoms suggest a more consequential wound to the engine.
While a water pump can begin this process, it is actually the overheating afterwards that does the damage.
Aside: I'm sorry that your 2017 is ending in this manner. Best of luck in 2018. Sincerely.