They definitely can ruin a bathing suit, especially if you have sensitive skin. A friend of mine, who is fair haired and has skin allergies, got them in her suit while snorkeling. She tried rinsing it out and washing it in the washer but no luck. She even tried washing it at home, even tried hot water at some point. I think she also tried vinegar. The suit kept causing a rash, so into the trash it went. DH and I went through a school of baby ones, not quite larval. DH had shaved that morning and has sensitive skin. He had a slight rash and tingling on his face. That's the only time we have ever experienced anything with jelly fish. We could see these and probably could have avoided them by going around or going deeper. We didn't know what they were. Sea lice might not be very visible though.