It looks like the water pump kit for a Johnson 15 should be $20-$40. Installing the kit is relatively simple and quick, however, when was it changed last? Biggest concern in my opinion is if any of the bolts are frozen. I have a Yamaha 150 that I religiously change every other year because..1. I never want a problem out on the water and 2. I put anti seize on the bolts to keep them from becoming frozen.
If it has not been done for a long time.. bolts may be frozen so I would consider taking it to marine mechanic.
Good luck and tight_lines