I'd have a look at these cottages.
Ardmaddy Castle Holiday Cottages, Oban - EmbraceScotland UK
They have there own slipway, they are at the head of the sound of Jura. Its very sheltered if the weathers a bit rough, if its calmer you are only 10 minutes from Easdale etc via Cuan Sound. They also have moorings. Would recomend Caddleton if there's a big group of you, or Craiguillean for a smaller party, 10 yards from the shore!
Alternatively you could also look at these cottages,
Self catering accommodation Oban, Seil Island, Scotland
They also have launching and a pontoon. Again its at the top of the sound of Jura, again 10 minutes by Cuan Sound to get round to Easdale but has the advantage of the more sheltered waters when its a bit rough out in the Firth of Lorne.