The J-class were the fastest submarines in the world when they were laid down for the Royal Navy in 1915: 84 m long, 1,210 t surfaced and 1,820 t submerged, driven by three diesels for 19 knots on the surface. Six survivors of the class were gifted to the Royal Australian Navy in March 1919, sailed out with the depot ship HMAS Platypus, arrived in Sydney on 15 July 1919 and were based at Geelong from 1920. Post-war budget cuts saw them paid off at Western Port on 12 July 1922, and in January 1924 the Melbourne Salvage Company bought the flotilla for scrap.

J2 was launched at Portsmouth on 6 November 1915 and commissioned on 1 June 1916; in June 1917 she took part in Operation BB in the North Sea and may have damaged U-99. Her Australian career was brief and mostly alongside. Stripped at Williamstown, she was scuttled on 1 June 1926 in 120 feet of water and forgotten until the Geelong Skindivers Club relocated her on 10 February 1974.

The 'Broken Sub' earns its nickname: the hull fractured about 5 m aft of the conning tower during scuttling, and the two halves now lie at different angles. The forward section lists roughly 45 degrees to starboard, its bow modifications and four torpedo tubes exposed where the bow casing failed, while the after section sits on its keel running north–south with the stern gradually being ground into the surrounding reef. The break is the feature of the dive: you can hover at the open cross-section and look straight into the pressure hull, seeing frames, bulkhead remains and the engine bed without entering.

Penetration is possible here but involves twisted, jagged plate and total darkness, and is only for divers with overhead training. A memorial plaque at 36 m recalls a diver who died on the wreck. Marine growth is heavy – sponges, ascidians and zoanthids – and the conning tower carries the usual dense cloud of bullseyes and butterfly perch, with crayfish under the casing and occasional blue devilfish.

J2 is the least visited of the four offshore submarines because it lies beside the main shipping channel and pilot boarding ground; charters dive it only at slack water with no ship movements due. That also makes it the most atmospheric. It suits experienced deep divers comfortable with a shot-line descent and ascent, and it is a rewarding second submarine once J4 and J5 have been done.

Boat only. Charters leave from Queenscliff Harbour (Red Boats, Dive Victoria Group) or Portsea Pier (Dive Victoria Group, Bayplay); drive Melbourne–Queenscliff via the Geelong Ring Road and Bellarine Highway (about 1 h 40 min) or Melbourne–Portsea down the Mornington Peninsula Freeway (about 1 h 45 min). Private boats launch at Queenscliff (Larkin Parade) or Sorrento ramps; the wreck lies a few kilometres outside The Rip, so time the run through the Heads for slack or a following tide.

Boat time from Queenscliff is roughly 20–30 minutes and from Portsea 30–40 minutes in a fast RIB, longer in a displacement charter boat. Gear is loaded at the harbour; there is parking at Queenscliff Harbour and along Point Nepean Road at Portsea.

Fully exposed Bass Strait. South-westerly swell is the killer: most charters will not run on a forecast over about 1.5 m, and anything from the south or west over 2 m makes the site undiveable and the passage through The Rip dangerous. Northerly and north-easterly winds flatten the sea; a calm morning after several settled days gives the best visibility (often 15–25 m in autumn and winter).

Surge is felt on the wreck in any swell over 1 m. Tidal streams outside The Heads are weaker than inside but still run; charters aim for slack water at Port Phillip Heads or a favourable tide for the transit. Winter westerlies close the site for weeks at a time.

The wreck sits in or near the shipping approach and pilot boarding ground, so a shot line, surface cover and a delayed SMB are mandatory.

Shot is normally dropped near the conning tower. Descend to the break about 5 m behind the tower, look into the cross-section of the pressure hull, then follow the forward half (which lies over at about 45 degrees to starboard) to the bow torpedo tubes. Return past the tower to the stern, which is worn down into the reef, and back to the shot.

Maximum 39 m on the sand; 20 minutes bottom time, ascend the shot with a DSMB as backup.

Within recreational limits on air or EAN28–32 but an SMB, redundant gas (pony or twinset) and a computed deco plan are expected; many divers run it as a light-deco dive on nitrox with a 50% stage. Shot line descents and ascents only – free ascents in the shipping approach are not acceptable.

Dive Victoria Group (Portsea and Queenscliff) – scheduled double-dive charters most weekends in season, weather permitting; Red Boats (Queenscliff) – regular weekend and weekday charters; Bay City Charters (Portsea/Queenscliff) and Chimney Rock Charters (Queenscliff) – regular weekend charters; Bayplay (Portsea) – occasional advanced charters. All treat the site as a deep/advanced dive and run it on the calmest days of the week. Dived less often than J4/J5 because of its position beside the shipping lane; charters need slack water and a clear shipping window.