Re: 1974 115hp Mercury - with new waterpump.
It should be a steady stream, if it's just a dribble it's not enough.
This is in the water. If you're using a flushing device like a set of "muffs", make sure your water supply is turned on high enough that a bit of water spills out of the muffs when the motor is running at idle. This ensures that the suction side of the water pump is completely flooded and is not sucking air.
BTW never take it above idle speeds out of the water, it's bad for the engine internals.
One way to tell if you've got adequate cooling is to feel the exhaust cover on the Port side of the engine, it should be cool at idle, or no more than slightly warm to the touch after running a while at speed.
It it's quite hot or you can't hold your hand on it, you're not pumping enough water and need to check things out further.
HTH............ed