I have to admit, I did not like the naval aspect of Rome II that much. I found the naval battles in Empire, Napoleon and Shogun 2 to be better - especially the ones in Fall of the Samurai. Luckily this is a period that is devoid of big naval engagements. The Mediterranean was in the control of the Roman navy at the time, and most barbarian peoples lacked the technology and funds to launch navies of their own. Perhaps towards the very end of the period you had the Vandals disrupting the area severely with sea raids - especially the time they sacked Rome under Gaiseric in AD 455, but I think that's slightly beyond Attila's timeframe. Most of the time the Romany navy just patrolled the seas across the Eastern coast of Britain against Saxons, or up the Rhine or Danube rivers.
Also, as mentioned earlier - the Roman ships of the time were a bit different to those used at the time of Marcus Agrippa and the Battle of Actium in 31 BC. Here's a modern reconstruction of one such warship from the period -