It all depends on the Civilization, terrain, and your technology level.
Ships, regardless of their ocean-traversing ability, can cross ocean as long as it is within their Civ's cultural borders. Thus, if there's an island separated from your city by one tile of ocean, you could access it as long as your culture enveloped the ocean tile.
The Portuguese Empire's Unique Unit (UU) is the Carrack, which replaces the Caravel. It has one cargo space, which can be used to ferry a Settler over to an island previously inaccessible.
Lastly, the Galleon is a ship with 3 cargo spaces, enough for a Worker, Settler, and military unit, which is accessible to all Civilizations with Astronomy. The Dutch Empire receives the East Indiaman instead of a Galleon. The East Indiaman has 4 cargo spaces and can enter rival cultural borders without an Open Borders treaty, like a Caravel/Carrack.