Sounds like he was using ground effect.
Though the effect is available, I can't imagine a reason to fly low enough to use it in a P-51 on a fun flight with no need to get somewhere. With a non pilot passenger, he would already be impressed at the speed of the aircraft, so the small ground effect increase would be meaningless. I would also think that no P-51 pilot would be deliberately taking a passenger that close to the water, there is not reason.
As the non pilot seemed to be flying, I can imagine the pilot distracted, providing verbal guidance to the use of controls etc. and "head in" for a crucial period. A non pilot would be ineffective at maintaining altitude over a featureless surface, and the P-51 could develop an awesome descent rate with few cues. I opine that this accident could have had it's fate sealed several hundred feet up.