Many Thais I know about cannot swim. I understood this thing as mostly people from countries with islands can swim and those that have huge land areas cannot. Not always the case. But the locals who can, in Thailand and Philippines, will jump into the sea not only in the bath suit, but in T-shirts or even jeans, especially women. It is more than obvious who is who, I mean if you want to distinguish foreign female tourists (all in bikini) from those local.