Oceans

This is one of those posts where you ask yourself ‘do I comment on the spelling, explain why oceans have different names, or ask whether he’s called Thumby because that’s the shape of his head?’

Oceans do have borders. An ocean is any body of water over oceanic crust. One example of this neat little trickery is that the Caspian sea is landlocked, but it is still an ocean. With the ocean being defined, the more obvious borders are areas where the oceanic crust ends. The less obvious ones are where the oceans seem to "change" in temperature, by currents, and politic/science. There is a lot of "Nuh uh, it ends here" with them, so it's more of a gradient of it becoming "less Atlantic Ocean" and "more Artic Ocean".

He knows too much, take him out

By that logic countries don't have borders either. Except in cases of rivers or other bodies of water, which isn't many of them.