If the players happen to get the little boat wet, it will rapidly expand into a full-sized rowboat, which can easily carry four people in full adventuring gear. Once dry, it returns to its original, non-encumbering size. Of course, things don’t become dry as soon as they are removed from water. It might take a few hours on a hot summer day, or even several days if the weather is less favorable. The party may be forced to leave it behind and retrieve it later.
If the characters do bring this with them, be sure to note who is carrying it, and take appropriate action if they, say, walk beneath a waterfall, or get hit by a blast of water while fighting an elemental. The suddenly expanding boat may destroy any fragile equipment stored in the same container as it.