According to The National Herald (Εθνικός Κήρυξ)

During the swims, a main boat with the tour guide on board follows our swimmers from a safe distance and at low speed. The boat is steered by an experienced local skipper, who has excellent knowledge of the local seas and their peculiarities. At the same time, a small inflatable boat or kayak with a lifeguard on board follows the swimmers more closely.