Besides the obvious dangers of sunburn Greek beaches also occasionally present other dangers ... certain times of the year such as late August, the jelly fish manifest themselves. Not Portuguese Man o' War but smaller stinging ones which hurt and can ruin your day. Jelly fish are called 'Tsouk-tress" and are small clear, diaphanous, and see-thru, they are hard to spot in the surf. Usually there are a lot more than one.
Those small black balls with long needle like thorns that like shallow water where they cling to rocks and wait for you to step on them are called Sea Urchins or Achee-noose in Greek.
Rocky shorelines and some few beaches in Greece have such sea urchin infested rocks that you will have to cope with. You can generally spot the sea urchins and try to avoid stepping on them.
Wave walkers for the rocks and the sea urchins are a good idea. Plastic sandals work too.
Be considerate sea urchins - you'll be glad! Sea urchins probably have no idea we exist and seem to live in a dimension all their own as you'll agree if you've ever seen those time-lapse films - they actually have places to go and things to do.