I have heard of new impellers not sucking in water from a hose when freshly installed. Its kind of like the system needs to be primed. The problem is that when it doesn't suck the water in, it fries the new impeller.

Dont know how old your boat is and if you run in salt water. If you do run salt water...pull the supply hose that attaches to the gilmbal. Adapt something from the hardware store to accept your garden hose. By testing this it will eliminate your water flow from the drive.
If you have good water at that point, pull off the hose adaptor from the gimbal. Check if theres a restriction where the hose goes thru the gimbal held into plave with a plastic wedge. It is simular to what your looking at when the drive is off on your bravo bellhousing that the drive attaches to.

Real crappy design from merc. The housing will corrode where the hose attaches to the gimbal. Had that problem and drove me nuts for 2 weeks.
Good luck!!!

How old is the inlet hose to the pump? I have seen them deterioriate to the point that they collapse internally and will not allow the pump to draw water.

????????
Is this the same pump?

Don't quote me on this... but I think you have the hoses backward? I believe, the intake hose from the drive goes to the top off the pump and the outlet is on the bottom? Don't quote me on this, at work and going from memory. If you did hook these up backwards and did not grease the heck out of the new impeller when installed, it is probably fried and housing badly scored. Only takes a few seconds of running dry to ruin them. I made that mistake once, housing got so hot it CRACKED! (Just took a few seconds)

Just double check... I might be all wet on this one. Good Luck

Zack