It's strange there are so many coastal routes. Shipping virtually anything by sea has been cheaper than moving it over land for a long time, and that probably includes troops. I would have expected roads to connect coastal settlements inland, not along the coast.
Settlements tended to concentrate along the cost and other water edges, for reasons of food availability, soil fertility, and access to the water base transport you mention. Existing local routes between them would have been used and upgraded as needed rather than building new links except where the time+capital expenditure made it worth while.
Settlements concentrated along the coast were mostly colonies of former naval powers, like Greek states, Phoenicians, and Carthage. These liked to settle close to shore but weren't connected by roads due to political reasons until submission to Rome.