He is famous for locating the wrecks of several ships lost during World War II. Mearns and his company, Blue Water Recoveries Limited, are in the Guinness World Records for the deepest shipwreck ever found, the German blockade runner Rio Grande, which was located at a depth of 5,762 metres (18,904 ft).
In 2008, Mearns led a search team to find the Australian cruiser HMAS Sydney and the German auxiliary cruiser Kormoran, which both sank following a mutually destructive engagement off Western Australia in 1941. Prior to finding HMAS Sydney, Mearns said that it was, in some ways, "bigger than the Titanic" because of what it meant to Australia. "Nothing comes close to the Sydney."
On 15 March 2015, Mearns jointly announced with Oman's Ministry or Heritage and Culture the discovery of a shipwreck from Vasco da Gama's 1502-1503 Armada to India. The wreck is believed to be the Portuguese nau Esmeralda that was commanded by Vicente Sodré the maternal uncle of Vasco da Gama. Having sunk in May 1503, the Esmeralda is believed to be the earliest ship from Europe's Age of Discovery ever to be found and excavated by archaeologists. Mearns published a paper, with co-authors D. Parham and B. Frohlich, on the discovery in the International Journal of Nautical Archaeology.
He has been responsible for the discovery of 24 major shipwrecks, including:
- the British Battlecruiser HMS Hood
- Lucona, sunk in 1977 by a bomb planted by Udo Proksch, as part of an insurance fraud
- Derbyshire, an ore-bulk-oil carrier which disappeared in 1980
- Mearns, David and White, Rob, 1945- Hood and Bismarck. Channel 4, London, 2001.
- Mearns, David L., The search for the Sydney. HarperCollins Publishers, Pymble, N.S.W, 2009.
- "'I'll find HMAS Sydney'". news.com.au (Sunday Telegraph). 2006-11-12. Retrieved 2008-09-01.
- Fraser, Andrew (21 December 2009). "Discovery proves maligned navigator got it right". The Australian. p. 3. Retrieved 21 December 2009.
- Smith, Aaron (4 November 2010). "Shipwreck finder gets honorary OAM". Australian Geographic. Retrieved 5 November 2010.
- "Live video feed of sunken Japan WWII warship Musashi by Paul Allen research team". 13 March 2015.
- "Vasco da Gama shipwreck Esmeralda near Oman from Vasco da Gama fleet".
- Mearns, David (14 March 2016). "A Portuguese East Indiaman from the 1502–1503 Fleet of Vasco da Gama off Al Hallaniyah Island, Oman: an interim report". International Journal of Nautical Archaeology 46 (2): 1–21. doi:10.1111/1095-9270.12175. Retrieved 1 April 2016.
- "Modern king of the shipwreck hunters". Herald Sun (Melbourne). 18 March 2008. Retrieved 14 June 2011.