The photo that shows the water having already receded and the folks strolling unconcerned on the beach is very interesting..they don't even appear to be looking out curiously at the lack of sea. I don't mean to be a Monday morning quarterback or to insult the dead, but it does seem as if the sight of that receding water would make people almost involuntarily run for their lives.
I'd put the % of the tourist population in Thailand that knew that rapidly receding water to a great distance offshore was a sign of an impending tsunami at maybe 2%.
Certainly, when I was very young, I learned that receding water like that meant a tsunami was on the way. But in today's world of careerist knowledge bases lacking vast areas of what used to be considered standard practical life knowledge, what, maybe 10% max, of the population would know this? I'm perhaps being too generous.
Since most of the people there were probably tourists, how were they to know what a "normal" extreme high or low tide was supposed to be in the area?
The locals should (and some few of them did, apparently) know better.
The thing I noted is that the first photo seems to have been taken from an elevation of maybe 25 feet above beach level, but that the later ones are right on the beach.
It seems as if this couple went down to the beach to get a better look.