A good century after Julius Caesar came to Britain the Romans came to the Estuary.
The legate,Titus Flavius Vespasianus aka Vespasian, whose legions maybe invested and captured Exeter in AD50, went on to be a Roman emperor and a good one too. Barbara Levick has written a biography of him, (Vespasian, Routledge, 2005) in which she writes that the campaign to conquer the South West was, "a combined land and sea operation with the troops being supplied by the fleet at such bases as Hamworthy in Poole Harbour and Topsham on the Exe."
If Dr Levick is right the Roman fleet, the Classis Britannica, was sailing, or rowing, up and down the Estuary from the earliest days of the conquest and Topsham was visited by Romans perhaps even before Exeter had been besieged. For three hundred and fifty years or so after this, Roman ships came and went to and from Topsham.
From the beginning Roman ships' captains must have needed pilots to guide them in and around the banks and channels of the Exe. By chance in York there survives an altar inscription that shows that a high ranking river pilot,gubernator, was employed by the VI Legion to bring ships up the Humber and the Ouse and no doubt the legions at Exeter too would have employed pilots to ensure that precious supplies reached the army.
At the time of Caesar's invasion the ships of the Roman fleet were famously ignorant of the movement of the tides. But by the time Roman ships were navigating the Exe they had probably got the hang of things.