seven mile beach is probably the best beach I have been to in SA, highly recomended. The really soft sand beach is Gunyah beach, I got severely bogged there by myself once!
Your'e right about the camping, they want you to camp in the camping spots which aren't right on the beach, but the one at the end of seven mile beach is fairly close to the water. Also, if you read the signs the greenies put up, they actually permit fires on the beach between high and low tide marks all year round, except on total fire ban days. Good fishing on seven mile beach, especially for whiting. There is a place near coffin bay itself where you can dive for scollops, but haven't found that place yet.