Brit_12687

I am taking a cruise from Florida to Caribbean (Cozumel & grand caymans) in July. I am a US permanent resident (I have a green card) and a British passport. My question is if anyone knows if the passport needs to be valid past a certain date I read that it needs to be 6 months past the trip date but then I also saw that Mexico and the grand caymans only require a non expired passport.