having "someone else" do it is:

1.) having to wait unitl they can "get you in" and if you didn't buy the boat there it could be a long wait.
2.) paying $125+ for a job that takes $40 in parts and 30 minutes of your time at most.
3.) KNOWING it was done right because you did the work yourself. The last place that changed my impellor left my speedo pick up hose between the nosecone and the housing so it became squished and did not work. Didnt figure it out until I changed it myself.
4.) I can do it in about 15-20 minutes now and I have the peace of mind every year that I have a new impellor.
5.) Its really not a big deal, almost as easy as changing spark plugs. If it was complicated, or if there was anything you could really screw up or if your simply not mechanically inclined whatsoever, then yes have some one else do it.

My reasons for doing yourself.