A half tide dock is a partially tidal dock. Typically the dock is entered at high tide. As the tide ebbs a sill or weir prevents the level dropping below a certain point, meaning that the ships in the dock remain afloat, although they still rise and fall with the tides above this.

What is a half tide?

: the time or state halfway between flood and ebb.

What is a half tide reef?

the state or time of the tide when halfway between high water and low water.