Lewis & Short

naumăchĭārĭus, a, um, adj. [naumachia],

  1. I. of or for a mock sea-fight: pons, Plin. 16, 39, 74, § 190.
    Hence,
  2. II. Subst.: nau-măchĭārĭus, ii, m., a combatant in a mock sea-fight, Suet. Claud. 21.