Killbear, Sandbanks and Mara are my favourites. They all have long sandy beaches with gradual depth. You have to book early 5 months in advance, for Killbear and Sandbanks though.
 

I have been to the beaches in Sandbanks and Wasaga. Both of them have nice shallow beaches that area ideal for swimming. Sandbanks Provincial Park have campgrounds that you can reserve but Wasaga Provincial Park doesn’t. Most people who go to Wasaga opt for cottages or private campgrounds nearby. There is also a lot of picnic crowd in Wasaga.

Where is the best locations to camp that have power and running water? 2 hrs from Toronto max

Puberty Provincial Park – Dunes or Burley campground are both close to an amazing beach on Lake Huron. 
Further north along Lake HUron is a Inverhuron Provincial Park with a nice beach. 
MacGregor Point provincial park has a nice beach but small in size for the size of the park and camping sites. 

Meant Pinery!!! See above

Sandbanks is amazing! The park staff are great and I’ve never had a problem finding parking! The campsites are also great! They are close to the beach and there’s always something to do! 

I love Grundy, no moterized boats makes for beautiful clean beaches

My favourite will always be Bon Echo Provincial Park. 

Killbear Provincial Park…sites just steps from the beach

Sleeping Giant Provinical Park has 3 campgrounds and lots of back country camping locations – I’ve been to Marie Louise Lake and would 100% recommend it if you’re up in north western ontario!

Fitzroy provincial park has two beaches! A larger more public one and a smaller more shallow beach for little ones or quieter times.

News Provincial Park on the north shore of Lake Superior. The beach is over 1km long with fantastic hiking trails that pop out to other beaches.

Charleston Lake has my favourite camping with beach access! Sandbanks is also a great option, just often has a lot more traffic.