We’ve lived in San Diego for the last 16 years. Unfortunately, yes, the seaweed has become pretty bad all along the Southern California and Baja Mexico coastlines. We’ve been experiencing significantly warmer water temperature for the past few years and the kelp has really flourished in it. If you went to the beach in the La Jolla, it seems to me to be worse there. I think because it collects in the cove. I’ve found Pacific and Mission Beach to be better. The beach in front of the Hotel Del Coronado is also quite nice, I believe they clear the kelp from the beach, but not the water.
As far as water temperature goes, my 12 year-old surfs and she gets into the water without a wetsuit year-round. I’ve never been able to get her to wear a full wetsuit, she feels too constricted in it. Most of her friends don’t wear wetsuits either. They all have a tradition of going for an ocean swim on New Years Day. Last year, they all swam for hours without wetsuits. But again, the water temperatures have been significantly warmer. The ocean temp has been over 78 degrees here over the summer and those temperatures have lingered well into the fall.
Another suggestion for a day off would be Catalina Island. You could do Sea World during the day then drive/take the train up to Dana Point, spend the night there and then take the Catalina Express to Avalon the next morning and spend the day there. This is my go-to trip for my usually land-locked visitors. There are no real waves in Avalon, but beautiful clear blue waters. You could rent lounge chairs at Descanso Beach Club and/or do the ropes course. Another really fun thing is to rent a golf cart and explore Avalon. We travel a lot, but Catalina Island is still one my daughter’s favorite places in the world.