Yeah, sounds like a good idea- the moon is often underused in videogames, and I'm not aware of any game that does this. It's sort of like the way some games have shops that are only open in the daytime, bars at night etc, except more natural. Would work well in a Tomb Raider game too. You could have secret areas open up, and the peril of being stuck when the tide comes in (in the North West of England, this is a serious thing).
So when's the game coming out? ;)