Unfortunately lots of potential reasons.
First, almost hate to ask this and I'm not being insulting (I've done this myself) I'm assuming you checked the really obvious like checking to make sure the seacock is open?
This is the raw water pump
IE the one that uses a rubber vane impellor? When he pulled the old impellor out were there any vanes missing? They often break off and jam in the system somewhere and block water flow.
Is the pump higher than the water line? If so, any air leaks
in the hoses between the water intake and the pump will keep the pump from self priming. Even if the hoses weren't removed sometimes messing around with the pump you can bump a connection and break the air tight seal.
Is he sure the new impellor is OK? Is the pump OK? Can he remove the cover, turn over the engine
and confirm that the new impellor is actually turning? This does beg the question, why did he change this impellor? Just routine and time for a new one or was something happening with that engine?