This object appears as a wooden box that measures 12 inches long, 6 inches wide, and 6 inches deep. It weighs 4 pounds and floats. It can be opened to store items inside. This item also has three command words, each requiring a Magic action to use:
First Command Word. The box unfolds into a Rowboat.
Second Command Word. The box unfolds into a Keelboat.
Third Command Word. The Folding Boat folds back into a box if no creatures are aboard. Any objects in the vessel that can’t fit inside the box remain outside the box as it folds. Any objects in the vessel that can fit inside the box do so.
When the box becomes a vessel, its weight becomes that of a normal vessel its size, and anything that was stored in the box remains in the boat.
Statistics for the Rowboat and Keelboat appear in the Player’s Handbook. If either vessel is reduced to 0 Hit Points, the Folding Boat is destroyed.
If either boat form is damaged (not destroyed), do I repair it as if it were a normal boat, so with carpenter's tools or could this also be done with the Mending-Cantrip to the boxed form itself (1 foot long, so within the measures of the Mending-Cantrip)?
If I were DMing I'd say you could do either.
Has built in rulings for being destroyed but does have the fun image of a thief using it as a bonus action.