Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
nătātor, ōris, m. [nato], a swimmer, Varr. L. L. 5, § 93 Müll.: pugnat in adversas ire natator aquas. Ov. R. Am. 122; id. H. 19, 70; Stat. Th. 10, 497.