A half tide dock is kind of like a bathtub for boats! Imagine filling up your tub with water until the water level reaches about halfway up the sides of the tub. That's what a half tide dock is like.
But why only fill it up halfway? Well, this type of dock is usually built near a body of water that has changing tides, like the ocean. When the tide comes in, it raises the water level in the dock until it's full. When the tide goes out, the water level drops back down to halfway.
Boats can use the dock when the water level is at its highest, but when the tide goes out, the boats won't be stuck in the dock because the water level will have dropped enough for them to safely sail away.
So basically, a half tide dock is a special kind of dock that uses the changing tides of the ocean to help boats safely and conveniently enter and exit.