Swimming with a mermaid costume and monofin is both fun and works as a form of exercise.
Around Sweden, there are swimming schools and bathhouses with courses and mermaid training.
The mermaid suits are made of high-quality and chlorine-resistant swimsuit material. To swim with your suit, you also need a monofin for the suit. Remember to put on your mermaid costume in direct connection to the pool/water. It is not durable or made to move on the ground with, so your suit and fin will last a long time.