As for all Nismo parts are a factory parts and have factory part numbers, I doubt it would void your warranty. Go to the parts dept at your dealership and check this out yourself, don't rely on your salesman's word.

Besides the filter gets mounted on the back side of an alum wall, that replaces your airbox. You would have to go into deep water to need to worry about sucking some water into the motor.

Good luck with this.