City in the Sea

A city in the sea shows the land-dweller that people can live in the sea;
they just need some way to breathe underwater.  All of the people who are
living in the city in the sea were born able to breathe underwater, and their
oxygen won't run out; but when the sea people try to walk on land their
oxygen runs out until they can get back to the sea.  So, the land people
don't know anything about the sea, and the sea people don't know anything
about the land. So they are never going to meet each other's race until a
land person or a sea person finds a way to breathe either in the sea or on
land.  Then, they can share stuff about cities in the sea and the land.
So, finally both the land people and the sea people can be friends and
share what they have done and stuff that they need to work on.  Now, you
just have to decide whether you want to meet the other race or not