The Gladiator 2 trailer launched yesterday (you can watch below if you've not yet seen it) and while it looked to be a great spectacle, some people found it to hold a few historical inaccuracies already. Like with his Napoleon film, it seems Ridley Scott is more going for a great cinematic experience than an authentic historical retelling.
However, if one of the things you questioned was the water battle which sees one gladiator torn apart by sharks, you might want to hear about the naumachiae or naval games. These games would see an arena filled with at least several feet of water, and see gladiators fight each other on board ships mimicking that of the Roman navy.
Emperors Claudius, Caligula, and Julius Caesar all were reported to have organised these naval games. Claudius' celebration of his civil engineering works saw 20,000 men involved in the battle, and Caligula took over a small lake with ships the size of a Boeing aircraft.
There are also claims that Heliogabalus filled an arena with wine rather than water, but that is likely a myth. There's no mention of sharks swimming in these naval battles, as it would have been terribly difficult to transport them, but we suppose we can allow that small inaccuracy for the movie.
