It's kind of a Mexican thing. We noticed it last time on a beach further south, well armed marines patrolling folks in bathing suits. When we asked, we were told it was to make us feel safe. We would have felt safer if we didn't run into so many of the guys with guns hanging out at the Deposito (beer store)...
This is why when the fireworks started we stayed inside. We had already heard a few disconcerting explosions through the week. So it just seemed prudent.
But then the sky lit up. Really lit up. And it took a moment to realize that the fireworks were going off on the breakwater, really, really close by.