Is this a natural waterfall or pumped water?
If it is pumped water you have a fountain and 680.51 applies
680.51 Luminaires, Submersible Pumps, and Other Submersible Equipment.
(A) Ground-Fault Circuit Interrupter. Luminaires, submersible pumps, and other submersible equipment, unless listed for operation at 15 volts or less and supplied by a transformer that complies with 680.23(A)(2), shall be protected by a ground-fault circuit interrupter.
If this is a natural pond and waterfall, it is not clear you need a GFCI for an underwater light, only a receptacle. See article 682