What counts as ocean and what you observe as ocean are two different things at times.

You can be in the middle of the "ocean" with no land in sight, yet be in meadows, swamps etc for example because that's what exists 3m below the surface. Sometimes the transition from land biome to ocean biome is quite abrupt and close to shore, leading you to seeing serpents close to it.

I've also got a kracken close enough to one of my bases that i can build a ramp out to it. technically its in the "ocean" but its within 20m of the shore.