I would permit flexible connections to help control damage were rigid connections may be more subject to serve movement during storms. I will search the NFC, because I recall reading a rule in another NFPA publication that allowed use of a suitable flexible cord.
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
[This message has been edited by Joe Tedesco (edited 08-17-2002).]