I am on an island and have optics about 200 turns in. I have a lot of the navigation research and many different ships. But there are different color tiles in the ocean that I can't travel across. For example, the tile looks black and shows ocean but only the light blue ocean, I can travel through. There is only one slim passage I can travel across. Why is this?
In order to travel the "dark blue" ocean tiles, and escape the limits of the "light blue" coast, you'll need Astronomy. Astronomy gives you the Caravel, a fast scouting naval unit that can cross ocean tiles, as well as allowing embarked units to travel on these tiles as well.
It's often the case that when you start on a remote island, it may be difficult or even impossible to find another landmass without Astronomy.