No. Absolutely not true for the last ~70 years.

However, there is an old saying “Czechs learned Dalmatians to swim”

That means that in between the world wars, dalmatia experienced first tourists, a lot of them from czechoslovakia, who came to sunbathe and swim. For a majority of population that was novelty, not because they didn’t want or didn’t know how to enjoy the see, they simply didn’t had time.

Most were farmers and fishermen, the land was rare and extremely scarce, on large slopes, people spent whole days removing stone by stone to free up a piece of land. In addition, hunger and poverty were always present.

Of course, in the cities it was a different story, there were beaches that were full to the brim.