This position is challenging because the surf board must not be moved enough to tip it over completely. This requires a lot of balance by both partners in addition to synchronized, gentle movements.
The position is most easily approached by beginning in waist deep water. This allows the partners to get into position more easily than in deep water. Ensure you follow all standard surf board safety - including a back-up plan for if you both topple into the water mid-intercourse.
The nature of the position adds an element of excitement as it may mean semi-public sex. For those without a surfboard, other aquatic sex positions can help fulfill the novelty of pleasure in the water.