I am not an inspector but I would not ask one to change requirements in this section.
In the NEC Handbook they give an explanation of this rule, it is an opinion not an official interpretation.
Handbook Commentary after 680.23(A)(5)
The reason for the 18-in. minimum submergence requirement is to reduce the likelihood that a person in the water and hanging onto the side of the pool directly over the fixture will have his or her chest in line with the fixture. This section covers fixtures that have been investigated and found acceptable for use where a person's chest may be directly in front of the fixture. The highest level of leakage current in the pool coming from a wet-niche fixture with a broken lens and bulb is found directly in front of the fixture.
Sounds like a good reason to follow the code.