This thread is halachic speculation at its worst.
We have one person inventing that swimming together is “lo sikrivu”. Then someone else is making up parameters for this “issur” (comparing it to the dinim of sleeping in same bed). Then we have someone claiming it is kalus rosh.
This is absurd. Lo sikrivu is only what Chazal say is lo sikrivu.
Even if it were to be lo sikrivu, there is no reason to assume it would have the same parameters as sleeping in the same bed. Touching a woman is also lo sikrivu, yet a father is allowed to touch his daughter over their entire lives.
The only issur in this thread is baal tosif.