The tiles adjacent to me are ocean within his borders... his (supposedly coastal) ships are entering ocean squares; however, Civopedia clearly says they cannot enter ocean squares. Perhaps 'pedia is wrong here...
The bottom line is that his ships are moving into my territory and back into his while mine can't. Asymmetrical rules like this need fixing; why is it sane for his ships to be able to sail from my coast back to his, when my ships can't do the same? Changing (formerly ocean) tiles two squares from land into coastal tiles is only one of the possible solutions...
Another option is to allow symmetrical movement for combatants or ppl with open borders. If we can code it to allow him to return from my territory, we can code it to allow my ships to enter his.
I wasn't telling how I
wished it should work, I was explaining the logic behind the present rules. I was explaining that it wasn't a bug but a result of the game rules. The game rules aren't my choice, so don't blame me.
The game-rule reason why he can enter those ocean tiles and you can't is because he culturally controls them.
You could ask why the rules are as they are. There are actually some very good reasons for that.
-If tiles far from the coast could be entered by ancient age ships, then large bodies of water could be crossed early in the game which would be uninteresting. Thus only tiles adjacent to land tiles can be accessed early in the game.
-Some sea food resources are 2 tiles from the coast and they need to be accessible. So the rule that when you culturally control a tile, then it is available for your ships was created. This way your work boats can enter and improve those tiles.
-If you could also enter hostile foreign culture ocean tiles, then that would make travel between continents easier and that is against the wishes of the game creators, thus they limited the movement into those tiles.
-Allowing movement into enemy culturally controlled ocean tiles and not into the ocean tiles which are culturally controlled by someone with whom you have closed borders is not logically consistent.
The result of these rules is the rare situation that you encountered in your game. If you'd want to mod that situation out of the game, then I personally think that the extension of coastal tiles to 2 tiles from the land area would work best. It removes the distinction between coastal tiles 1 tile from land and culturally controlled ocean tiles 2 tiles from land. Of course, it would lead to easier movement between continents early in the game, but it would be more logically consistent. A different change would be to not allow ships into culturally controlled ocean tiles. That would stop use of ocean based sea food resources until after astronomy. That could also be interesting.
I'll bet that the creators of this AI-mod are not willing to go that far to remove the situation that you encountered.
... coastal waters have navigable features... open ocean simply has big waves which can damage small boats.