I wonder if it's possible to relate the tiles directly offshore of provincial shorelines, such that if, say, you owned Segesta and Arretium but not Massilia, you would be able to sail very fast (more than the standard coastal rate) from Roma to Segesta, but to go from Segesta to Tolosa would be at half speed or less because the coastline (and thus the safe anchorages, water supplies and victuals etc) would be owned by the enemy. Unless the enemy has a military access treaty with you.

Then that way, the scripted relations (like Massilia being an ally of Rome, Bosphorus being allied to Pontos) would be also used to reflect accessibility.

As I see it, the most farfetched part of the idea is the military access one. I think diplomacy is hardcoded, so. But yes, differentiating the coastal tiles according to faction also could be possible. Is it? Or is the sea untouchable?