Keep in mind that SD County spans many miles both along the coast but also inland. You have cities in North SD county like Oceanside and Carlsbad and go as far south as National City/Chula Vista/San Ysidro (and given your preferences for the ocean, you probably aren't going to want to live any further inland than beyond Escondido or El Cajon). I'm not an SD Expert by any means but as with many locations there are good and bad/grungy pockets to be aware of. Not sure if you noticed the last time you visited but homelessness is a problem particularly around downtown (last time I drove around there it felt like driving through nicer parts of Skid Row... these days, it's probably worse) - not sure what location in particular you were thinking about but just be aware of that... which I guess goes back to your 4th point of purchases being *less* expensive than in Boulder. What area(s) of SD are you specifically looking at again? Definitely use AreaVibes and GreatSchools to get a better idea of the particular areas you're considering. Even a cursory view via Google Maps/StreetView can give you an idea of the area (and I'm not just talking about the immediate neighborhood area but those major cross-streets outside of where you're looking)
That said, consider North SD County and even South OC for that matter - Oceanside, Vista, San Juan Capistrano and San Clemente are nice areas to live, and depending on what you want for housing, may meet your needs and still be close to prime surfing spots.
As far as moving further from your parents, well, not sure what I can say about that. Are they in their latter/senior years and unable to fly or have medical needs requiring extra care, etc? If so, and depending on how close you are to them (and how burdened you are to help) then that's definitely going to be a factor in all of this. Maybe if those things are true, you can consider/plan to rent for X years so you're not glued to the location while working to figure out the elderly care piece if that's a future concern.