system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5 how a moon pool (aka wet porch) works, and why a ship with one doesn't sink.

I know it must have something to do with water pressure, but was hoping for a concise ELI5 answer to understand it more easily. Thanks.
gpt
There are a few different kinds of moon pools.  The easiest one to understand are basically a hole in the floor above the waterline of a drilling platform or ship with multiple hulls that lets you look down into the water.

You're probably thinking of moon pools below the water line though.  Imagine taking a cup, turning it upside down and then pushing it under water.  You will have trapped some air inside the cup.  The water wants to get in the cup, but it can't make a lot of progress because the air is taking up the space (the air does compress a little).

Now imagine drilling a hole in the side of the cup while its under water.  Obviously the air will leak out through the hole, and the water will rise to a bit above the drill hole.  

This is how moon pools work, except that the hole in the cup is an airlock that keeps all of the air from escaping.  While the air is kept in place, the water cannot enter.  If the moon pool were turned sideways, or had a hole, it would indeed fill with water.