if one undresses to go to the mikvah or take a shower, or go swimming, does he have to make a brocho on his ציצית when he puts them back on?
According to the Ashkenaz custom, a blessing is not made on the tzitzis after wearing them following a short break, such as for using the bathroom or taking a shower. Only after a lengthy break, such as for the purpose of a 2 hour game of tennis (or similar event), would a new berachah be recited upon wearing the tzitzis again.
According to the Sefardi custom, a new berachah is made even if the tzitzis is taking off for a short time.
This cleavage of custom follows the dispute between the Shulchan Aruch and the Rema (Orach Chaim 8:14) concerning this question. Although the Vilna Gaon rules in accordance with the Shulchan Aruch, the regular custom (Ashkenaz) follows the Rema.
Some are careful to have intent when they make the blessing in the morning that the blessing should apply even after they take off the tztizis are wear it again.