Info summarised from
The Roman War Machine, by John Peddie:
It can be surmised that Caesor's first reconnaisance mission to Britain (AD 54) included approximately 250 ships, including approximately 100 troop transports, several scouting ships, assault landing craft, administration vessels and dedicated warships.
The next year he arrived in force with about 800 ships in total.
So, they obviously had a decent sized navy. It was basically a support arm of the army, and each ship's crew was treated as a century of the army.
Appart from ballistas and other light siege weapons, it is possible these vessels were armed with siphon operated flamethrowers (utilising greek fire); certainly such weapons were in use by the time of Constantine.
From
Decisive Battles of the Western World, Vol I, by J F C Fuller (info is a bit sketchy because I need sleep and can't be bothered studying):
The Greeks raised approximately 324 triremes and 9 penteconters to meet an Egyptian invasion force, circa 480BC; the Egyptian navy was approx. 1,200 warships and 3,000 transports (or at least, that was the claim), but only about 200 warships were involved in the invasion.
It seems the numbers involved there are unusually high (probably due to a Greek alliance vs Egypt); a later battle (circa 420 BC) between Syracuse and Athens shows only about 80 ships a side.
Hope something here is intelligible and semi-useful.
