Sporades archipelago is home to a wide variety of marine wildlife. Seals and dolphins are commonly encountered during boat trips around the islands of Skiathos, Skopelos, Skyros, and Alonissos—the only four inhabited ones from 24. Alonissos Marine Park is one of the best places to watch marine life in Greece. It's also the largest marine protected area in Europe and the first site of its type in Greece. Its fauna includes one of the largest seal species, the endangered Mediterranean monk seal, that inhabits the islands of Gioura or Piperi.
You can visit the Marine Park and then go island hopping by boat. However, keep in mind that not all Sporades residents are open to tourists. It's better to plan your visit for the summertime since the islands can only be accessed by boat and ferries, which run irregular schedules in the winter. You can also swim and sunbathe on the pristine beaches of Sporades islands during this time.