Born in Norfolk and raised in Great Yarmouth before being evacuated to Oxford during the Second World War, he went to a sea training school (TS. Mercury, Hamble) at the age of 14.

Larn taught himself to dive in 1947, using a German-made Dräger U-boat escape set in the River Thames. He then joined the Merchant Navy where he served his apprenticeship as a deck-officer with the South American Saint Line and eventually became 2nd Mate.

In 1950 he transferred to the Royal Navy where he stayed for 22 years. As a Chief Petty Officer Mechanician/diver he served in Korea and participated in technical diving involved in recovering ram-jet un-manned target drone off Malta being dropped from a helicopter up to five miles offshore, and with Petty Officer John Guppy was possibly the first divers in the Royal Navy to operate using diving apparatus from helicopters, long before SAR divers. Expeditions all over the world.

In 1957 he became a BSAC member, and served as BSAC Deputy Diving Officer in 1961 and 1962. Larn was also among the instigating members of NACSAC,the Royal Navy Sub-Aqua Club, which was established in the early 1960s with Lieutenant Roy Graham as its first chairman, and Larn as its Diving Officer.

One of the club's first major projects was to send a team of divers to the Isles of Scilly off the coast of Cornwall to find an historic Royal Navy ship, like HMS Association, a 90-gun ship of the line lost in the great naval disaster of 1707.

In 1964 about ten NACSAC members, including Larn, arrived on Scilly – thought at that time to be only the second group of divers ever to come there. Their initial dives sparked off a long chain of navy visits that continued for four years. In 1964, 1965 and 1966 the divers could only get out to the Western Rocks, but hardly around the Gilstone Ledge, where a later expedition managed to locate the wreck of HMS Association in 1967; Larn was not present in 1967 having been given a "pier-head jump" to HMS Bulwark in Singapore.

In 1972 Larn left the Royal Navy as a Chief Petty Officer to pursue a career in private business. He founded a commercial diving training centre in Falmouth, which aimed at improving the training standard of professional deep sea divers in the oil and gas offshore industry. In 1976 he established the Charlestown Shipwreck and Heritage Centre which grew out of his own collection of sea artifacts and which he ran until 1998. Living in Charlestown for 31 years, he was joint owner and curator of one of the largest collections of shipwreck artefacts on public display in Europe. After retiring from the shipwreck centre Larn and his wife Bridget moved to the Isles of Scilly in 1998, where they had first lived from 1986 to 1991 when they set up and ran the Longstone Heritage Centre.

Together with his wife he has written over 56 books and countless articles on maritime history and archaeology, shipwrecks and the sea. Their 'Shipwreck Index of the British Isles', a work with 45,000 ship details for the Lloyd's Register of Shipping, was used by the Royal Commission for Historic Monuments to establish the National Maritime Record, followed by a similar record for the Scottish and Welsh Governments.

In retirement, Larn is still active in diving, mostly in the Isles of Scilly. His other interests include figurehead and ship carving. When in 2007 the council of Scilly commemorated the 300th anniversary of the great naval disaster of 1707, Larn was among the principal organisers and also gave a public lecture, as did Dava Sobel, author of Longitude, and Sir Arnold Wolfendale, the 14th Astronomer Royal.

Honours

Besides receiving awards from diving and maritime history associations, Larn was made a Cornish Bard by the Gorsedd of Cornwall at Redruth in 2006. In the Birthday Honours 2009 he was made an Officer of the Order of the British Empire (OBE) for "services to nautical archaeology and marine heritage". Awarded the USA's 'Knight of Mark Twain'(1970)