J7 was the odd one out of the class: built at Devonport Dockyard and launched on 12 February 1917, she was completed to a modified design with the control room and conning tower moved further aft and a submerged displacement of 1,760 tons, 60 tons less than her sisters. Commissioned into the Royal Navy in September 1917, she came to Australia with the flotilla in 1919 and was the last of the J-boats in commission, serving as a depot and training boat at Flinders Naval Depot before being paid off. Sold with the others, she was stripped and on 6 August 1930 was sunk as a breakwater for the Sandringham Yacht Club, where she has remained ever since – a strange, low silhouette built into the marina wall.

There is not a great deal to dive. The hull sits in 1–2 m of water with much of it awash at low tide; the casing has largely gone, the conning tower was removed, and what remains is the long pressure hull with its hatch openings, frames and rivets, colonised by mussels, weed and small bay fish – leatherjackets, crabs, sea stars and the occasional seahorse in the marina growth. It is best treated as a heritage snorkel at high tide rather than a scuba dive, and because it lies on the yacht club's lease, permission should be sought at the office.

Its value is historical. J7 is the most accessible J-class submarine in the country, visible from the foreshore path, and together with J3 at Swan Island and the four Graveyard boats she completes the story of Australia's first submarine flotilla. Divers working through the Victorian J-class set usually pay her a visit, photograph the hull from the breakwater and then do a proper dive on the Sandringham breakwater and pier 155 m away, which offer far more life in the same shallow water.

It suits snorkellers, historians and completists; it is not a destination dive.

Sandringham Yacht Club is at Jetty Road, Sandringham, 16 km south of Melbourne's CBD – take Beach Road and turn into Jetty Road; paid public parking is at the Sandringham foreshore car parks and along Jetty Road, with the club's own car park for members. The submarine forms part of the marina's outer breakwater on the club's private lease, so ask at the club office before entering the water; the usual approach is to walk along the breakwater and slip in beside the hull at high tide. Sandringham Pier and its breakwater, a recognised shore dive, are 155 m to the south and share the same parking, toilets and cafes on the foreshore.

Nearest fills are at Diveline (Frankston) or Melbourne bayside dive shops.

Inside the marina the water is flat in almost anything, but the site faces west across the bay, so a strong westerly or south-westerly drives chop and dirty water into the breakwater and makes the shallow hull surge; easterlies and calm mornings give the clearest water. High tide is essential – at low water the hull is awash and there is barely a metre to work in. Visibility in this part of the bay is 2–6 m and is worst after rain when the Elster Creek and stormwater outfalls discolour the water.

Marina boat movements are constant on weekends; a dive flag and a spotter on the wall are sensible.

At high tide, enter beside the exposed hull from the breakwater, follow the pressure hull along its length looking at the hatch openings and the collapsed casing, then swim back on the other side. Combine with the Sandringham breakwater 155 m south for a longer, more rewarding dive.

No operator dives the J7; it is a heritage curiosity visited by the occasional snorkeller with yacht club permission. Melbourne bayside dive shops (Diveline, Scuba Culture) run the neighbouring Sandringham breakwater and pier as shore dives.