If It's Southern California, Why Haven't We Seen Kara & Co. At the Beach Yet?

Well they only use Southern California for the exposition shots now after the switch from CBS to the CW, so realistically they couldn't have a beach episode. However, in Season One Episode Ten (Childish Things; the one where Winn's dad[Toyman] breaks out of prison), there is that scene on the boardwalk and I believe (not sure) there's a few shots of the beach; unfortunately, not a beach day like OP was hoping for.
Sounds like a fun concept though!

"The only problem I see, Stewart, is that they would be freezing in the Vancouver weather. "

Yes, of course! I was really just joking - and chuckling at the idea of Kara and Alex trying to surf! Hey, I have no idea if Melissa and Chyler can surf (But come to think of it, maybe their stunt doubles can! )

"Since they start filming in July it might be warm enough that the actors would still be comfortable.'

Yes, it would be in the 70s (20-25 C) then, but the water would probably still be too cold to swim in comfortably. A pool party would probably be a better idea!

"How about a Hawaiian themed party that Kara has to leave from to get a Hawaiian shirt rip?"

Or a "luau" at a Polynesian restaurant - I imagine they have those in Vancouver.....

The only way something like this would be possible is if it were in the first couple of episodes of a season. Since they start filming in July it might be warm enough that the actors would still be comfortable. The issue with having it in the first couple of episodes is that those are a set up for the season and I don't see a beach scene fitting into that. I suppose they could film a scene in July and use it in a later episode but realistically probably no beach scene happening. How about a Hawaiian themed party that Kara has to leave from to get a Hawaiian shirt rip.

The only problem I see, Stewart, is that they would be freezing in the Vancouver weather.

Argo

Bahahahahahaha! Have you ever tried to do a shoot in Vancouver in 5 C to 10 C temperatures, rainy, blustery weather? It's the equivalent of doing a beach scene on the Thames in early March. Now, imagine doing it in a bikini and flip-flops.