Is it permissible to swim on Shabbat, and what Halachic issues may be entailed?
Before proceeding to answer this question, we must clarify that we deal with a private pool, where only men or only women swim. Mixed swimming is strictly forbidden any day of the week, both on Shabbat and weekdays. Our entire discussion here refers only to separate swimming.
Furthermore, we discuss here only a pool situated in a Reshut Ha’yahid (private domain), where one does not violate the prohibition of carrying on Shabbat. One may certainly not swim in a pool situated in a public area, as he would be in violation of Shabbat by the fact that he carries water as he swims.
In addition, we deal here with a pool containing cold water. It is forbidden on Shabbat to bathe in heated water, even if the water is only lukewarm. Hence, the question regarding swimming on Shabbat arises only when dealing with separate swimming in a pool situated in a private domain and that contains only cold water.
The Shulhan Aruch (Orah Haim 339) rules that, strictly speaking, it is permissible to swim on Shabbat in a pool that is entirely surrounded by walls, as our pools are. This is as opposed to swimming in a river, which is forbidden on Shabbat.
Nevertheless, all contemporary and recent Halachic authorities unanimously rule that one should not swim on Shabbat. As several Poskim note, even if swimming is intrinsically permissible, it gives rise to a host of potential Halachic problems. For example, one may not squeeze water from wet hair, wet bathing trunks or a wet towel on Shabbat. The work Yalkut Yosef (Shabbat, vol. 2, starting on p. 7) lists numerous other Halachic complexities, as well, that arise from swimming on Shabbat.
Additionally, swimming on Shabbat is wholly inconsistent with the aura of Kedusha (sanctity) and spirituality that is to characterize the Shabbat experience. Leaving aside the potential Halachic pitfalls of swimming, and assuming that one ensures to swim in a manner that could, technically, be deemed permissible, it undermines the sacred nature of Shabbat observance, and for this reason all authorities, without exception, forbade swimming on Shabbat. These Poskim include Hacham Ovadia Hadaya (in his Yaskil Abdi 4:1), Rabbi Moshe Stern of Debereczyn (in his Be’er Moshe) and Rabbi Yishak Weiss (in his Minhat Yishak). This is also the ruling of Hacham Ovadia Yosef.
Summary: Strictly speaking, it would be permissible to swim in a pool on Shabbat under certain conditions. In practice, however, the Halachic authorities forbid swimming on Shabbat due to the potential Halachic problems that arise, and because swimming is inconsistent with the sense of sanctity and reverence with which we must treat Shabbat.