Some people have seen the sea, but others haven't. The sea looks beautiful on a fine day. But it can be very rough when there is a strong wind.
The sea is very large. There is more sea in the world than land.
If you have swum in the sea, you know that the sea is salty. Do you know the Dead Sea? It is so salty that you can't sink when you are in the water. And fish can't live in it!