Are there sharks on Greek beaches? Well no, we are not in Australia or the Caribbean. The truth is that it is not that there cannot be Sharks in the Mediterranean Sea but they are extremely rare and in this country there is only one shark attack registered more than 100 years ago so I think you can swim calmly.
But that does not mean that you should have beware of other animals or other dangers that are much more real and common. There are jellyfish, sea anemones and in certain places sharp rocks so I would tell you that a good pair of flip flops or rubber sandals are an accessory that you cannot ignore. In addition, this type of footwear will also protect you from the famous Scorpion fish, whose stingers usually have good aim and are fatal.
Advice? Flip flops at all hours of the day. You never know that it may be stalking you on the ground.