Ski Island is great because you can anchor you boat right by your sight when the water is up and there are 3 great coves to ride in. If you want to go up the Sac arm, Gooseneck cove has some campsites with a pretty good shoreline with small rocks - not so muddy or big rocks like most of the lakes shoreline.
Neither of these places have running water but they do have nice pit toilets (if you can call a pit toilet nice). Our family of 5 plus dog boat in camp at Shasta all the time - you just have to be organized.
This is a pick of a Ski Island campsite last July.