Halkidiki and Thassos are best planned as a land-based northern Greece water trip: dive Kassandra wrecks and Sithonia walls, snorkel Ammouliani, Diaporos, and Thassos coves, then build dry days around Mount Athos cruises, Petralona Cave, and mountain villages. The underwater style is Mediterranean rather than tropical: excellent clarity, rocky reefs, Posidonia meadows, octopus, nudibranchs, groupers, amberjack, and occasional dolphins or monk seals. Halkidiki is the easier training and day-boat hub from Thessaloniki, while Thassos adds island atmosphere, marble beaches, and boat-access snorkel bays. The sweet spot is June and September to early October, when water is comfortable, operators are active, and the July-August heat and crowds ease.