Shower off after swimming to prevent swimmer’s rash.
Swimming in any kind of water (fresh, salt, chlorinated) can produce a skin rash for some people. These different types of dermatitis may have different causes.
Hot tubs (poorly maintained ones) may have the bacteria Pseudomonas aeruginosa in their water. Fresh or salt water rashes come from parasites or their larvae. All of them can cause itching, redness or blisters right away or within a few days of swimming. Some rashes (like duck worms or sea lice) are worse under a swimsuit because the larvae are crushed more between fabric and skin.
The best way to prevent such rashes is to rinse off in a shower both before and after swimming. Most pools and beaches have showers available. When possible, remove your suit when you shower (or rinse under it as well as possible if not).
If that doesn’t work, hydrocortisone cream to the rescue.