If my dog has arthritis

[Myrina]

4. Treat her the same as you would an arthritic human ...

... use moderation, according to the weather (heat/humidity) and how she seems to be feeling.

And a big YES to the beach. Swimming is great exercise for arthritic dogs, my old man Hank has always loved to swim - great way to get his exercise in summer and not overheat, and now that he's a senior, it's gentle on his old legs/hips.