Found it in the 2002 NEC here:
555.13 Wiring Methods and Installation.
(A) Wiring Methods.
(1) General. Wiring methods of Chapter 3 shall be permitted where identified for use in wet locations.
(2) Portable Power Cables. Extra-hard usage portable power cables rated not less than 167°F (75°C), 600 volts, listed for both wet locations and sunlight resistance, having an outer jacket rated to be resistant to temperature extremes, oil, gasoline, ozone, abrasion, acids, and chemicals shall be permitted as follows:
(1) As permanent wiring on the underside of piers (floating or fixed)
(2) Where flexibility is necessary as on piers composed of floating sections
You can end up with quite a large loop when at high tide since the low tide for a floating dock or pier requires the longest lenght of cord for the boardwalk ( average 6 feet here - yours may vary)