I'm 99% sure you have an alpha drive based on previous threads. It's not a difficult job to change the impeller on an Alpha, and it's certainly a DIY project. There's a common misconception that having the pump in the drive leg versus on the engine (like a Bravo) makes the job really complicated. It's not true, IMO (I've replaced both Alpha and Bravos impellers).
Here's a link to the procedure:
http://www.mercstuff.com/waterpumpinst.htm