[L124C]

I just rebuilt the raw water pump on my Yanmar 2GM20. The pump worked fine when I took the boat out, I probably motored about 30 minutes. Went for a sail, and when I fired the motor up to come back, no water was coming out of the exhaust. I took off the raw water strainer top, ran the motor, and still no dice. I ended up disconnecting the water hose where the pump supplies water to the motor and the pump was then able to supply water again. I motored back without any problem. I assume there must have been an air blockage somewhere in the line (probably at the impeller?). If so, how would it develop after initially running for half an hour? If not, what might have caused it. Is my method the best way to fix the problem should it happen again?