Q. I’m thinking about working as a baby swim instructor but have to go on an 8-day retreat to complete the training. I will be only listening to instructions and be in the swimming pool. Can I do this over Shabbos if I don’t have to write? Can I do it or teach over the 9 days if necessary?

A. Shulhan Aruch (O. C. 339: 2) permits in principle to swim on Shabbos in a pool that is entirely surrounded by walls, as most of our pools are and you can therefore carry in the area.
However, many Poskim in our days prohibit not only swimming but just soaking even in an unheated pool. (Igrois Moshe E.H 2: 13, Minchas Yitzchok 5: 32, Beer Moshe 3: 56, Shemiras Shabbos Kehilchasah 14:12; Yaskil Avdi 6:1, et. al.)
Different reasons are mentioned for this prohibition such as; One may improvise or fix swimming or floating paraphernalia. (Kitzur ibid.). Or one come to squeeze out water from a towel, wet bathing suit or hair. Even taking a cold shower is not allowed. Playing by a pool causes great loss to the intended holiness of Shabbos. However, when the immersion is done for a mitzva purpose, such as tevila in a mikva, it is allowed. Poskim also permit when the bathing is needed for the ill (Yeshuos Moshe 3: 35, Piskei Teshuvos 339: n. 4).
In principle, one may attend in case of need a class or university lecture during Shabbos. Conditionally if, one will not come to touch muktza articles, carry or do any other even Rabbinical melochos. (See question 156).
Therefore, Horav Shlomo Miller’s Shlit’a opinion is that in case of need (parnassa), since this is only a one time occasion, you may join that instruction group. However, you may not swim on Shabbos, you have to be careful with drying hair or squeezing water of the bathing suit. (The Rov maintains that it is allowed, since people usually are not makpid or care if their bathing clothes are wet).
You must prepare well, all the needs to observe Shabbos properly, including timers, candles, kiddush, chalos etc., to maintain the becoming spirit of kedushas Shabbos.
The Rov maintains that the above could be done even in the nine days in case of great need, otherwise it should be avoided.

Rabbi A. Bartfeld as revised by Horav Shlomo Miller Shlit’a