Could HMS Hood have pioneered water armour?

Should the Royal Navy have introduced water armour in HMS Hood so later ships could benefit from the experience?

Only if the system had matured in time for Hood's design and construction. The technology was still developing and may instead have been relevant to later Admiral-class ships.

Operational experience in Hood could have helped later development, but the choice was not available on demand. A protective scheme has to be ready early enough to shape internal volume, structure, and displacement before construction is too advanced. The system was still developing and might have become practical for one of the later projected Admiral-class ships rather than Hood herself. The exchange shows how a sound technology can miss a particular ship because its engineering timetable does not match the shipbuilding timetable.