If you will be in "the south end" of Copacabana (I assume Posto 6) you can swim with OTREINO (http://www.otreino.com.br). They are a tri group that swims two or three times per week in Posto 6. Just go there, introduce yourself and enjoy. They have kayaks and charge like 5 USD per session I believe to support that.

Thanks for this.
Are you aware of any pools? I’m happy to take a cab wherever.

First option is to check with your hotel. Some of them have super nice 25m pools available (Sheraton, Hilton, etc).

Botafogo (http://www.botafogo.com.br/sedes.php?cat=oclube) in its Mourisco Mar unit has nice 25m and 50m pools close to Copacabana. Not sure what you need to do to access the pools, you need to check with them and they may charge you a fee.

You can also check on gyms that have pools available. There are many in the south region of Rio, but you will need a cab ride as Copacabana gyms are usually old and don't have good pools. Ask your hotel concierge.

I would also recommend to stop by in one of the Tri clubs that have tents on the beach and askl. I'm Brazilian but don't live in Rio.