An assembly includes a fin and a fin mounting box for a water sports board. The mounting box is mounted to the board and has an outwardly facing channel formed therein. The fin includes a fin base and a body extending from the base. The fin base is shaped to be received in the channel. The channel includes channel engagement formations, and the fin base includes corresponding fin base engagement formations which engage the channel engagement formations when the fin base is inserted into the channel so as to retain the fin to the mounting box. The assembly includes a locking tab received in the channel to prevent movement of the fin base in the channel and disengagement of the corresponding channel and fin base engagement formations.