In short
A single coastal line, east to west: Chora Sfakion ↔ Loutro ↔ Agia Roumeli ↔ Sougia ↔ Paleochora, plus Gavdos offshore. April-October season. The key point: Loutro and Agia Roumeli have no road — the boat (or the E4 path) is the only access. Sfakion ↔ Loutro: 20 min, €11. Sfakion ↔ Agia Roumeli: 1h, €17.