The family had a guide with them, so this was NOT an unsupervised visit. The guide is named here:
MUEREN AHOGADOS EN CHARCO DE DAMAJAGUA TURISTAS NORTEAMERICANOS
They also appear to have had life jackets & helmets. Certainly the photo taken in the morgue shows a helmet. Caution: the picture is not horrendous but if you're a family member or close friend be advised this is a photo of the deceased.
Link removed out of respect to the family. - Gregg, Mod.
This happened Thursday afternoon and certainly Puerto Plata had really heavy rain for an hour or so on Thursday afternoon. It could be that they were already in the pool when the heavens opened.