Re: 45 hp mariner outboard not working, replaced impeller
First: If the motor has been sitting, the outlet tube could be clogged. I took a piece of 14 gauge wire (stranded/insulated-trailer wire) and ran it through the outlet all the way out the pick up tube while I had the lower unit off. I was amazed how much crap came out. Bugs and ESP. dirt dobber LOVE outboard motors!
Second: If you try that and still get nothing you need to rebuild your water pump. (It's the assembly the impeller is sitting on top of.) Not too hard, you need to get a kit off here or at a local dealer.
I'm personally betting on the dirt and bugs. It's a common thing, esp. if you are in the south.
Best of Luck!