A glass bottom boat is a boat with sections of glass (or other suitable transparent material) below the waterline allowing passengers to observe the underwater environment from within the boat.
Glass bottom boats are used almost exclusively for giving tours, as they are usually designed to allow the maximum number of tourists to view out the glass bottom and are not really suitable for other uses.
Typical tours include shipwrecks, reefs and other underwater flora and fauna.
Slang: The process of defacating over your partner with their face covered by a glass plate in order to see the inner workings of your sphincter.
