The impellar is located in the lower unit of your outdrive. It's a little tricky to remove though. Have you ever worked on an outdrive? Are you doing general maintenance or is there an overheat issue? These are very impt questions to know before I direct u further.
Also need to be sure this is an alpha drive we are talking about...
All Alpha 1's and Alpha 1's Generation II drives have a pump in the lower unit. Your motor also has a circulating pump on the front just like a Chevy in a car.
For the outdrive pump, I remove the drive from the boat and put it in an outdrive stand.
The drive splits in two pieces.
Once you remove the upper from the lower, you will see the water pump housing.
Google the subject and there might be some good diagrams and step by step instructions.