I'm off the impression that the Romans didn't travel by sea often, based on accounts of the cruising of the English channel; they seemed terrified of the sea. Not exactly sure why they didn't do this more often; maybe their strategy was to move by land and secure a spot channel as they went?
Sailing on the Mediterranean, which is very nearly an inland sea and has almost negligible tides, is an entirely different proposition from sailing on the Channel, which acts as a funnel for storms barrelling through from the Atlantic to the North Sea and vice versa. For an extreme example, see the Great Storm of 1987 [0], where winds reached hurricane force.
The Royal Yachting Association (UK governing body for all sailing sports) used to, and for all I know still does, regard the Med as inland waters, at least inasmuch as a dinghy-sailing instructor's certificate gained on the Med would not bear the coastal endorsement it would have had it been gained on the UK coast.
I don't know much about the Roman history, but a thought: There's a difference between crossing a sea and traveling on the sea skirting the coast (keeping the coastline always in sight).
Shipping large amounts of grain from Egypt was vital for the livelihood of the populace of Rome the city, so there was some maritime know-how in the empire.
