There’s another snippet in the Guardian booklet about the Roman world. “To the Romans, Britain was an exotic, terrifyingly remote prospect, across an alien sea with unpredictable tides”.
My guide to the tides around the Channel show them to be predictable, though there is a large tidal range west from Calais. Of course, the regularity of the tides there was unknown to the Romans.
The Mediterranean is almost completely landlocked, only open to the Atlantic at the narrow Pillars of Hercules. The Mediterranean is almost tideless. No wonder that the tides in the Channel scared the Romans.