You have a few solutions I can see:
Be more proactive. If the boatyard is busy and they can put you off as long as they need to they will. That way they can take care of their more demanding customers. So DEMAND better service.
If the above doesn't work, find another mechanic.
Learn to do the work yourself. Replacing spark plugs? My 11 year old can do that!
Of course if you bought a SAILBOAT you wouldn't need 16 spark plugs!