If you live near the coast they are the best, never too hot or too cold. The farther you go inland the hotter it can get during summer. Near the beach it's pretty much in the 70's most of the summer with some days in the 80's. Maybe once or twice during the summer it might hit 90. During the night low temps usually are in the mid 60's to low 70's. The temperatures during the summer are more consistent with the least variation than any other time of year. Inland areas can get into the 90's and low 100's during the summer. The ocean water is warm enough to swim in as well and very nice. The beaches are pretty much perfect during summer.
Summer is my favorite season in San Diego b/c the weather is perfect and the water is warm.
here is the average temps for throughout the year for San Diego, which is what most coastal locations would be:
http://www.weather.com/outlook/events/sports/wxclimatology/monthly/graph/92101?from=36hr_bottomnav_sports