Lewis & Short

nătātōrĭus, a, um, adj. [natator],

  1. I. of or belonging to a swimmer, that serves to swim with, natatory (post-class.): cortex, Isid. Orig. 17, 7, 27.
  2. II. Subst.: nătātōrĭa, ae, f., and nătātōrĭum, i, n., a place for swimming, a swimming-place, a bath; a pool, Sid. Ep. 2, 2: vade ad natatoria Siloe, Vulg. Joh. 9, 7; 11.