Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
sub-năto, āre, v. n., to swim beneath (post-Aug. and very rare): pars subnatat unda Membrorum, pars exstat aquis, Sil. 14, 482: currus bijuges alii subnatant, App. M. 4, p. 157, 6.