It all comes down to how deep the water is where you are.

You can be out in the middle with no land yet still not be in ocean due to land under you appearing as a biome instead.

Swamp and Mistlands seem to have very short reaches into the water typically. Meaning you can get very close to shore yet still come up ocean.

Odder yet is when another biome is close by and it shows up instead of swamp even if you are by the swamp.. another biome is basically showing up under it/around it.