I may be naive. This seems silly to me. I have never heard such a thing for any sport I have ever coached or participated in. Muscles being relaxed is not bad. I would be worried more about getting proper sleep and eating right for competition. If anything, having a good time with team mates and relaxing will help calm any pre-competition nerves and anxiety. I would limit hot tub use, being in one too long is not good for you anyways. Kids are active when they swim, even casually. I don't believe it would affect them so dramatically that it would ruin their athletic ability. They stretch and warm up before competing too. I have no proof either way with this discussion, just my opinion and limited knowledge of sports medicine. I would love to know the technical reason WHY , if in fact this is true.