Not a real big job. I've done it on my F115, alone, in about an hour. There are a lot of little washers and spacers that need to go on; take pics or make a drawing so you get everything oriented correctly and in the right order when it goes back together.
Ideally, you want the whole kit - impeller, gaskets, washers, housing, and base plate. Probably not needed, but since you've got it apart anyway...
This details it pretty well (though the pics seem to be gone - google should get you what you need, though):
Pics! Yamaha Water Pump Replacement F115 - The Hull Truth - Boating and Fishing Forum