While this boat is most likely to be from the bronze age there are examples from medieval times of this practice in ruins around the lake - notably in Ross Errilly Friary where live fish were evidently kept in the kitchen area having been transported to the Abbey from the lake.
The piece of timber  laying behind the vessel is the transom board, which would have fitted into the slot in the aft-most transverse timber and been caulked into place with moss and wax or fat.