I would straight up die if I went in the water without a wetsuit but I think being from Canada you'll be fine anywhere from Cayucos and further south. The northern areas and Big Sur have massive cliffs and very rough waters.

On the Central Coast, Avila Beach tends to be warmest.

Go to the ocean in San Francisco using the N Judah streetcar to the last stop. There you can dip your toes in the water to see if you find it tolerable. If it is, there are many beaches between San Francisco and Carmel. However, be aware that rip tides, sneaker waves and sharks can be dangerous.

Ocean temps vary but in San Diego in June they average about 64-65F (Scripps Pier area).
Every person's tolerance to heat and cold varies (heck I used to surf without a wetsuit at age 17 but now in my 60s I would not go in without one) so I think you will have to judge for yourself.

Possibly Monterey or Carmel on a very warm day. Malibu again, on a warm day. Best bet for swimming without a wetsuit would be San Diego.

You can rent wetsuits, but consider the cheaper and easy on/off lycra "stinger" suits. They are lightweight, and do give you some protection from colder water. They are easy to pack in a suitcase and very stretchy. We used these when snorkeling in Australia and they looked and felt great!

One option (no waves, but warmer water) is to swim in Mission Bay in San Diego. It's on the inland side of the peninsula where Mission Beach and Pacific Beach are located, so you can have both the ocean and the bay to choose from, depending on temperature tolerance.

Just throwing in a useless comment - I think you'll be fine with those temps. I learned how to swim as a child in the Puget Sound, and thought the waters at the beaches in the San Francisco area were fine. When as an adult I went into the Atlantic for the first time at Cape Hatteras, I was astounded at how warm it was.

Do watch for rip tides and those odd waves that can come barreling in out of the blue. Have fun.

Tomorrow morning (9AM PT) there will be a group of crazy geocachers swimming in the Pacific Ocean at Sunset Beach Oregon. I did it 3 years ago. I went in next to a woman in a bikini when I did it. The water and air temperature were @ 50*.
Probably tomorrow you will probably see video of a few people doing their annual "polar bear swim". I don't expect to do it again - ever.

janisj - LOL!!! I agree, but we kids didn't know any differently. And staying in the water was - eventually - less chilling than being onshore in a wet swimsuit with the cold ocean breeze. I didn't know anybody just floated around in the water until I got to the Atlantic and then to the warm waters of the Gulf. Energetic activity only..

I just looked at the logs and pictures from the geocaching event on geocaching.com. If you want to look, it is GC8GY93. Dan&Dawn log has a picture of more than 40 polar bear swimmers in the water. Less than half of the people in the water were geocachers. I have known Mrs.Rtreever for more than 10 years. This was the first time she went in the water on New Years Day. She posted a picture of the geocachers that were at Sunset Bay. After the "swim" they all went to breakfast at Tai's Dynasty in North Bend OR. Yesterday it was raining on the coast so everyone was wet in some way.

Wherever you swim in the ocean, make sure there are lifeguards and check with them about which area is safest.

That's a big if there are lifeguards. Most beaches I swam at in Northern California did not have them. However, do pay attention to the signage, and watch for "sneaker waves," that can catch you unawares; they are unexpected and very dangerous.

And be aware of any shark warnings.

Luckily they're also going to So Cal where we do have lifeguards.