I agree the immaculately groomed luxury beaches look inviting and safe... but if you ever look at the water I don't know how you could ever think it was safe or inviting... for years the water in SSL has reminded me more of a semi stagnant pond than an inviting place to wade, swim or even go near.
Maybe that night was the first time they had been over near the water? If it's dark you can't tell that it's gross. Or maybe they just aren't a very wimpy family and aren't terrible bothered by a little dirt.
When I was a kid one of my favorite places to swim was this big nasty man made pond full of all sorts of creepy crawlies. One minute I would be catching tadpoles, and the next I'd be jumping off the dock into the same water.