Your outboard runabout should make a good learner boat for nice days in the San Juan's, Kate. Don't try to turn it into the ultimate yacht, or anything that it's not, but rather find out what's needed to get the hull tight and rot free, and the propulsion dependable.
Be sure that you don't spend more on the fixup than it would cost to replace it with something better.
As for diving, all you need to get going in addition to your safety gear is a dive flag and an inexpensive ladder if the boat doesn't have any stern platform. Save the bells & whistles (except the one mandated by the CG) for later.
As for experience, start slow, take it out a little ways on calm, severe clear days and slowly learn your way around. Buy a chart book and figure out which way the buoys and day markers mark the channels. Learn about the currents. There are lots of places in the San Juans where you must pass on the correct side of buoys or you will find the bottom. I know that there are classes, especially this time of year, given by the local power squadrons. Check the board at your local marina, or at West Marine in Seattle or Anacortes. I'm not sure where you are, but my friends at Island Marine on Lopez are pretty fair and helpful.
The San Juans are one of the best places for small boating on earth. I wish our waters and facilities were 1/10 as friendly, though we have pretty good diving and beaching.
I'm off to beg the State Boating Harbor Officer at Maalaea to allow me an additional month in a temporary slip at double the normal rate, since there is a 12 year waiting list for permanent slips on Maui.
Aloha,
Jonathan
kalepa@maui.net