The UNESCO Convention is like a set of rules that grown-ups made to protect things that have been hidden or lost under the water for a very long time. This is called the underwater cultural heritage.
This can be anything really old, like ships that sank a long time ago, or maybe some old buildings or parts of cities that were once above water but have since sunk. These things are like history books that can help us learn about the past and how people used to live.
The rules tell grown-ups how to explore and study these things without destroying them, and how to keep them safe so they can be enjoyed by everyone. It's like telling kids not to break things in a museum, so that everyone can keep coming back to visit and learn from them.