Northwest Sardinia combines three complementary trip zones. Alghero is the practical hub for Capo Caccia boat diving, sheltered training water, a lively old town and direct airport access. Stintino adds the shallow turquoise setting of La Pelosa and shorter access to Gulf of Asinara operators. Asinara is the protected finale: a former prison island where scuba, snorkeling and land visits follow designated sites, landing points and authorized-operator rules. Underwater, expect limestone walls, arches, caverns, true overhead cave routes, coralligenous growth, red coral, gorgonians, Posidonia meadows, groupers, barracuda, octopus and lobster. The destination is not a liveaboard circuit. It works best as a land-based trip with local day boats, often split between Alghero and Stintino or Porto Torres. May through October is the broad water-season window, while spring and fall are better for long Asinara walks. Wind, especially on exposed western and northern routes, can force smart site substitutions.