Seiko Maru

A 5,386-ton freighter sitting upright on a sandy bottom with torpedoes standing upright in her holds, collapsible bicycles, and an accessible engine room — a well-rounded deep wreck dive.

Overview

The Seiko Maru was a 5,386-ton freighter, 114 meters long, launched in May 1940. On February 17, 1944, during Operation Hailstone, she was bombed by SBD Dauntless dive bombers from USS Enterprise, scoring direct hits that ignited a large explosion causing her to sink immediately. She rests upright on a sandy bottom with the bridge at approximately 23 meters, hold areas at 30—40 meters, and the keel at 49 meters.

The Seiko Maru’s holds contain a varied military cargo, highlighted by torpedoes — some of which are still standing upright in the positions they were stowed for transport. Collapsible bicycles and various other supplies fill the remaining hold space. The engine room is accessible and worth exploring. The upright position on a flat sandy bottom makes the wreck’s layout intuitive and navigation straightforward, creating a satisfying dive for those with the depth experience to reach her.

Site Information

• Location: Chuuk Lagoon, Chuuk State, Micronesia
• Entry Type: Boat dive
• Site Type: Wreck dive
• Difficulty Level: Advanced
• Maximum Depth: 49 meters
• Typical Visibility: 15-30 meters (50-100 feet)
• Current: Generally mild inside the lagoon
• Best Time: Year-round; December to April offers calmest conditions

Marine Life

The Seiko Maru’s upright profile provides vertical structure from the sandy bottom to the upper superstructure, supporting marine life across a range of depths. Hard and soft corals colonize the masts, king posts, and railings. Sea fans and sponges encrust the superstructure. Schools of fusiliers and jacks are present in the mid-water column. Glassy sweepers fill the engine room and hold interiors. Lionfish and scorpionfish occupy overhangs and dark recesses throughout the wreck.

Dive Profile

The bridge at approximately 30 meters is the shallowest major structure and a good starting point. From the bridge, divers can explore forward to the holds where the torpedoes and bicycles are stowed. The torpedoes standing upright in the holds are the wreck’s most striking feature. The engine room, accessible from the deck, provides an interesting interior exploration. The sandy bottom at just under 50 meters can be reached but offers limited return for the depth cost. Most of the dive should be planned around the 30-40 meter range where the holds, bridge, and engine room provide the most rewarding exploration.

Entry and Exit

Access is by dive boat from Weno-based operators. Entry is by giant stride. Descend along the mooring line to the wreck. The upright position and clear layout make orientation straightforward on arrival. Surface at the mooring line or deploy an SMB. Lagoon conditions are generally calm.

Tips and Recommendations

• The upright torpedoes in the holds are the signature feature — ensure your dive plan prioritizes visiting them
• The collapsible bicycles are a poignant reminder of the individual soldiers this cargo was destined for
• The engine room is accessible and interesting; allocate time for it on a second dive if the first focuses on holds
• The upright position on flat sand makes this one of Chuuk’s most navigable deep wrecks
• Nitrox or trimix is recommended for the depth; plan gas management carefully for profiles approaching 50 meters
• Carry primary and backup torches for hold and engine room exploration

Safety Considerations

The Seiko Maru is a war grave — removing any artifacts is strictly illegal under Chuuk State law. The torpedoes in the holds are live ordnance — do not touch, stand on, or disturb them under any circumstances. At just under 50 meters maximum depth, mandatory decompression may be required depending on profile and bottom time. The engine room is an overhead environment requiring wreck penetration training, redundant lights, and a clear exit plan. The upright position makes the wreck layout intuitive, but silt accumulation in enclosed spaces can reduce visibility rapidly with careless fin movements. Maintain proper trim and buoyancy throughout the dive.

Photography

The Seiko Maru’s upright torpedoes are her most compelling photographic subjects — their vertical stance in the holds creates powerful, unusual compositions. A diver positioned beside a torpedo provides essential scale. The bridge at 30 meters receives moderate ambient light and supports atmospheric superstructure compositions. The engine room interior rewards careful lighting with strobes. Powerful strobes are essential at depth to restore color across the wreck. The upright position and clear layout make pre-planning compositions feasible, allowing efficient use of limited bottom time.

Sources: Michael McFadyen’s Scuba Diving - Seiko Maru, trukwreckdiving.com - Seiko Maru, Dirty Dozen Expeditions - Wrecks Above 40m, Scuba Diving Earth - Truk Lagoon Wrecks. Last updated 2026-05-22. Correction 2026-06-16: maxDepth corrected from 40m to 49m (bridge ~30m, keel ~49m) per multiple sources; JSON depth also updated.