Honestly, it's pretty yucky to think that someone's dog is allowed to swim in a drinking water reservoir. There are some "grey water" reservoirs out there, but I'd not want my dog to swim in that water either.
Couple of options:
1) Laguna Beach coves allow for dogs to be on leash, but most of the smaller coves (Diamond Street for example) will find dogs running around off leash. Bring some fresh water for your thirsty dog to drink.
2) Trabuco Canyon river is flowing. You could go to O'Neil Park and let your dog run in the stream, or go out towards Holy Jim Canyon (Need a $10-$15 daily pass to do so) There's plenty of water out there.
3) In the RSM area at the end of Altisima Road there is a cul de sac to park at. Take a 15 minute hike up the hill (very well marked) and you'll find a seasonal pond near a cell tower. It's called "frog pond" by us locals as come late March there are thousands of frogs in the pool. Great for kids as long as they are well behaved and don't squish / manhandle the frogs. Dog swimming is a regular sight.
4) All along Santiago Canyon Road are places to park and walk into some of the free flowing streams.
In every case, watch out for budding poison oak, ticks, and be sure to get your dog bathed afterwards
My .02c
