Am I going to ruin my jewellery by wearing while I do any of the
following activities: swim in pools, swim at the beach, or take a
shower?

I have a 18 carat gold chain and a silver ring that are very valuable
to me. I hate taking them off, so I always keep them on. I've been
doing so for the last few years, and I haven't noticied any damage to
them yet. But I want to know if I'm going to cause them damage
eventually. I live in Sydney, Australia, if that matters.

Alle - Fine Jewelry
http://www.allejewelry.com/finejewelry.cfm?edit_id=10

"Never swim in a swimming pool or use a hot tub while wearing your
jewelry.  Chemicals such as chlorine can pitt your jewelry and can
damage white gold. Further, concrete (such as pool decks) are rough
surfaces.  Allowing gold jewelry to come in contact with rough
surfaces can cause scratches and mars."

use nail polish on your jewelleries before a swim , it will protect it.