A folding boat is always discovered as a small wooden “box”—about 1' long, 1⁄2' wide, and 1⁄2' deep. It does, of course, radiate magic if subjected to magical detection. The box can be used to store items like any other box. If the command word is given, however, the box unfolds itself to form a boat 10' long, 4' wide and 2' deep. A second command word causes it to unfold a vessel 24' long, 8' wide, by 6' deep.
In its smaller form, the boat has one pair of oars, an anchor, a mast, and lateen sail. In its larger form, the boat is decked, has single rowing seats, five sets of oars, a steering oar, anchor, a deck cabin, a mast, and square sail. The first holds three or four people, the second carries 15 with ease.
A third word of command causes the boat to fold itself into a box again. The words of command may be inscribed visibly or invisibly on the box, or they may be written elsewhere—perhaps on an item within the box. The words may have been lost, making the boat useless (except as a small box) until a finder discovers them (via legend lore, consulting a sage, the physical search of a dungeon—your imagination is the key).