A fin assembly includes a fin body and a fin holder. The fin holder has an elongated channel, a pair of opposing slots, a pair of front guiding grooves and a pair of rear guiding grooves. The pair of front guiding grooves are defined in the opposing sidewalls of a front section of the elongated channel at a horizontal level, and misaligned with a bottom of the elongated channel. The pair of rear guiding grooves are defined in the opposing sidewalls of a rear section of the elongated channel above the horizontal level, and are flush with the bottom of the elongated channel Besides, the fin holder defines in its rear wall a horizontal hole with a cross section large enough to cover a space between the pair of rear guiding grooves.