These mosaics of round pebbles are common in the Cyclades and the Dodecanese islands. On some islands, these pavings of smooth rounded stones were said to be designed that way so they could be easily made slippery as a defense against the pirates who raided Greek islands at many points in history. Now they are done for decoration, but on rainy days, the occasional tourist finds that they can still be a good defense against intruders.
By the way, without giving a spoiler, keep an eye out for a magical Aphrodite, Goddess of Love connection with this dolphin.... and you may even get a glimpse of the entire pantheon of Greek gods and Goddesses at the very end of the movie.