new water pump wont pump

I got a used 15 hp nissan early 90s is all i know about the year model, I was concerned about the water pump so i installed a new one and it will not pee. So i pulled the lower unit and cleaned the tell tail out with a tip cleaner was nothing in it re installed the lower unit and still nothing removed the lower unit again and removed the water pump to check if i could see a problem. i noticed nothing inspected the housing and everything looks good re installed the water pump. Thought that the pick up tube was not in place so i checked and it is seating properly and rubber gasket is good. so i thought i would check the pick up tube for leaks. I removed the power head and pressurized the pickup tube no leaks at all the tube holds pressure. Thought it might be the gasket under the power head being bad and applying exhaust pressure to the water pump system stopping it from pumping so replaced the gasket re installed all parts and still no water out of the pee hole. Removed lower unit again and looked everything over thought i would place the lower unit in a bucket with water level gust under the top of water pump and spin it with a drill. So i did and it pumps water out the top of the water pump re installed and still no water out the pee hole. i dont know what could be the problem please help.

If the pump is working (is the impeller key installed?), and the water pipe is connected and clear, you probably have a blockage in the power head. Did you try connecting a garden hose to the water pipe to confirm good flow in the power head?