This legendary Seiko Marinemaster Professional “Tuna” (Ref. SBBN031) is a purpose-built saturation diver engineered for real-world extreme use, featuring Seiko’s ultra-reliable 7C46 high-torque quartz movement, massive shrouded titanium case design for impact protection, bold high-contrast lume, and a professional-grade 300m water resistance rating trusted by commercial divers. The case measures approximately 47mm in diameter with a compact ~48mm lug-to-lug that keeps it surprisingly wearable despite its tool-watch presence, paired with a unidirectional dive bezel, day-date complication at 3 o’clock, and rugged Hardlex crystal. Built like a submarine and designed to outlast just about anything on your wrist, this is one of Seiko’s most iconic and collectible professional dive watches — a true no-nonsense tool watch with legendary durability and everyday usability.