Someone told me that if a person did not swim prior to the three weeks, he may not start swimming during the three weeks? Is there a source for this? (sources)
Although people say that one may not go swimming for the first time during the Three Weeks, there is no halachic source for this practice. It is therefore not considered a binding custom and it is permitted without Hataras Nedorim (Piskei Teshuvos 551:46).
One may swim during the three weeks, but not after Rosh Chodesh Av. Some however have a custom not to go swimming during the three weeks at all.
It sounds like those who allow one to "Continue swimming" is a subset of those who don't swim during the 3 weeks, but it is not the normal custom.
Swimming - Three Weeks (not including nine days)
Some have the custom to refrain from swimming during the three weeks. 121 However, the custom is to go swimming until Rosh Chodesh Av. Nonetheless, one should avoid swimming in dangerous places. 122
- Mekor Chaim 551:14:page 289, Orchos Rabbeinu 2:page 129:12. The Steipler zt”l did not change his clothing all week even during the three weeks except for Shabbos and his tallis kotton (Orchos Rabbeinu 2:page 130:17).
- Opinion of Harav Moshe Feinstein zt”l quoted in Moadei Yeshurun page 128:footnote 12, Halichos Shlomo Moadim 2:14:footnote 7, Teshuvos V’hanhugos 2:263, M’Bais Levi 13:page 22:4:footnote 5 quoting the opinion of Harav Wosner Shlita, Rivevos Ephraim 3:333, 4:135:14, 6:285:2, Shevet Ha’kehusi 1:169:4.
I have not seen a source for this in the Shulchan Aruch. It's possible this was instated because swimming brings joy, and it isn't appropriate to engage in joyful activities, with a limited number of exceptions, during the 3 weeks. According to the Shulchan Aruch the majority of the stringency's begin on the 1st of Av, notable exceptions being Orach Chaim 551.17 and 551.18. The R'aMA notes in a number of places the custom in Ashkenaz is to be stringent from the 17 of Tammuz (see Orach Chaim Ch. 551.2, 551.4, 551.9). I have not seen the Shulchan Aruch specifically reference swimming.