I do not know if I agree with the idea that the ships were small and the routes always costal.
Wrecks have been found in the middle of the Mediterranean, at locations well out of sight of land.
In the highly detailed story of the Apostle Paul’s voyage to Rome that ended with his being shipwrecked on Malta it is recorded the there were 276 people on board. This was a 60 AD cargo ship, not a passenger ferry. People were a secondary profit center. To carry 278 people in addition to it's primary cargo it would have had to been well over 50 feet long.
However the Antikythera wreck 120 years earlier shows that ships in the 160 foot length range were in use.
http://www.whoi.edu/news-release/antikythera-finds
Stunning Finds from Ancient Greek Shipwreck
New Antikythera Discoveries Prove Luxury Cargo Survives
Components of the ship, including multiple lead anchors over a metre long and a bronze rigging ring with fragments of wood still attached, prove that much of the ship survives. The finds are also scattered over a much larger area than the sponge divers realized, covering 300 meters of the seafloor. This together with the huge size of the anchors and recovered hull planks proves that the Antikythera ship was much larger than previously thought, perhaps up to 50 meters long.
"The evidence shows this is the largest ancient shipwreck ever discovered," says Foley. "It's the Titanic of the ancient world."
The shipwreck dates from 70 to 60 BC and is thought to have been carrying a luxury cargo of Greek treasures from the coast of Asia Minor west to Rome. Antikythera stands in the middle of this major shipping route and the ship probably sank when a violent storm smashed it against the island's sheer cliffs.
Given that they were out of sight of land, useing the stars for navigating would make a lot of sense.