You can also sail ocean tiles if your culture borders extend over them (or anyone's borders that you have Open Borders with).
This is what's called a "Culture Bridge". If you find yourself stranded in a small area with no way out (because you're completely surrounded by ocean), but you can SEE the edge of more coast tiles with a narrow strip of ocean at one spot, here's what you do. Found a city at the point of land just off that strip. Build an obelisk or temple or whatever to expand your culture. When your culture goes over the ocean tiles, your galleys can now sail over it to the coast beyond. (And, if you don't have Open Borders with anyone, then you are completely safe from any incursions from other civs!)
Wodan