Below roughly 150 metres, the Black Sea holds no oxygen. Around ninety percent of its volume is anoxic, a lifeless column that has preserved wooden hulls almost intact for centuries. Boats running Sunny Beach boat trips tours cross only the thin surface layer above it, freshened by the Danube and about half as salty as the Mediterranean. The sea is virtually tideless. That is why the jetties along this coast sit so low to the water, and why the bay stays flat through most of a summer morning. Storms arrive from the north-east in autumn; in August the swell rarely builds before afternoon. Departures gather at Nessebar South Port, on a peninsula that was an island until silt and stone joined it to the mainland. Thracians settled here as Menebria. Dorian Greeks refounded it as Mesambria around 510 BC, minting silver coinage by the fourth century. Rome, Byzantium and the Bulgarian tsars each took their turn. The Ancient City of Nessebar entered the UNESCO World Heritage list in 1983, and roughly forty churches survive in some form within forty hectares. The eleventh-century Church of St Stephen still carries more than a thousand painted figures. Fishing families kept houses of stone below and timber above, an Ottoman-era pattern repeated along the lanes. From the water, the ruined apses read as a low brick skyline above the seawall. North of the harbour the coast changes register. Sunny Beach — Slanchev Bryag — was laid out as a state resort in 1958 along eight kilometres of shallow sand, and now anchors the busiest stretch of the Bulgarian littoral. Beyond it the Balkan range ends at Cape Emine, a headland where the mountains meet the sea and the wind shifts without warning. Yachts crowd the marina at Sveti Vlas. Excursion traffic between Nessebar, Sveti Vlas and Pomorie has run in some form since the 1960s. Three cetacean species live in these waters — bottlenose dolphin, common dolphin and harbour porpoise — all protected, all occasionally surfacing off the bow of a Nessebar boat tour in the early hours. The appeal is geographic rather than ceremonial. This coastline is legible only from offshore: the peninsula in silhouette, the windmill on its isthmus, the cliffs at Ravda, the pale sweep of the bay behind. Wooden schooners rigged as pirate ships share the quay with catamarans and small fishing craft, a fleet assembled less by design than by demand. A day on the water covers ground no shoreline path can. Sunny Beach boat trips tickets are sold for that view rather than for any single monument, and the Black Sea coast holds its shape best when seen from a deck at anchor.