I have heard that instructional swim is allowed during the 9 days, but not free swim. I also know camps that do not allow any swim. Perhaps there are different opinions
Ask your Rav. In our days it was no swimming, bathing , shopping, handwork as knitting or tapestry, no music…in short aveilus. Today, with the doros getting weaker, many people ignore and do what they feel. Take out the kitzur shulchan orech and read what it says!