The naumachia (in Latin naumachia, from the Ancient Greek ναυμαχία/naumachía, literally "naval combat") in the Ancient Roman world referred to both the staging of naval battles as mass entertainment and the basin (or more broadly, the complex) in which this took place.
The first known naumachia was given by Julius Caesar in Rome in 46 BC on occasion of his quadruple triumph. After having a basin dug near the Tiber, capable of holding actual biremes, triremes and quinqueremes, he made 2000 combatants and 4000 rowers, all prisoners of war, fight. In 2 BC on the occasion of the inauguration of the Temple of Mars Ultor ("Mars the Avenger"), Augustus gave a naumachia based on Caesar's model. As cited in Res Gestæ (§ 23), he created a basin on the right bank of the Tiber where 3000 men, not counting rowers, fought in 30 vessels with rams and a number of smaller boats.
Claudius gave a naumachia in 52 AD on a natural body of water, Fucine Lake, to celebrate the completion of drainage work and tunneling on the site. The combatants were prisoners who had been condemned to death. Suetonius' account, written many years after the event, has them salute the emperor with the phrase "morituri te salutant" ("those who are about to die salute you"). There is no evidence that this form of address was used on any occasion other than this single naumachia.
The naumachia was thus a bloodier show than gladiatorial combat, which consisted of smaller engagements and where the combat did not necessarily end with the death of the losers. More exactly, the appearance of naumachia is closely tied and only slightly earlier than that other spectacle, "group combat", which did not pit single combatants against one another, but rather used two small armies. There again, the combatants were frequently those on death row and did not have the specialized training of true gladiators. Caesar, creator of the naumachia, simply had to transpose the same principle to another environment.