A water sport device with an adjustable boot. The boot is mounted to a support on which the user rides. The boot includes separate toe and heel portions that are adjustable to fit the needs of the user, and to also accommodate different users. One or more rails extend along the support and provide for adjustment of the boot. Each of the one or more rails includes an adjustment mechanism that can be rotated by the user to move the toe and heel portions relative to each other to adjust the boot size. Each rail may also include a locking mechanism to prevent adjustment of the boot. When engaged, the locking mechanism may prevent any adjustment of the boot, or may prevent enlargement of the boot but allow for additional reduction in boot size.