From the point of view of shipping losses, June 1943 – in English waters at any rate – was a quiet month with only two recorded losses. One of those was HMS Sargasso, a yacht requisitioned in 1939 as a danlayer.
At this distance in time, danlayer operations are somewhat obscure so it is worth explaining exactly what a danlayer is, or was. They were primarily small coastal fishing or other craft which were requisitioned the Second World War to operate in tandem with minesweepers, themselves often trawlers.
Minesweeper and danlayer personnel received a common training – there was an officers’ course, which took in Royal Naval Volunteer Reserve officers from Australia, Canada, India and New Zealand, as well as US personnel, Belgians, Dutch and Norwegians. For ratings there was a three-week ‘Hostilities Only’ course ‘so that they may take their places as efficient members of any minesweeping ship, whether she be trawler, drifter, dan-layer or fleet sweeper.’ 
The minesweeper-trawler went back to the First World War, but the danlayer is a specifically Second World War concept, and, as a consequence, lost danlayers are also a form of loss unique to the Second World War.
Like minesweepers, they were particularly prone to being lost to unswept mines on the periphery of the swept area, as in fact happened to HMS Sargasso. Girl Helen and HMS Elk, also sank in this way in 1940, but HMS Comfort was lost in a collision in the melee off Dover while coming to the assistance of torpedoed vessels in Operation Dynamo, 1940, and HMS Gulzar was bombed from the air, also in 1940. 
The Germans also requisitioned ships as danlayers in a very similar fashion during the Second World War and the Dutch coaster Seaham, ex Oceaan, was commandeered on the fall of the Netherlands for this purpose, serving as Wangerooge. Post-war she was returned to her owners and reverted to the name Seaham, trading once more with Britain. She was lost in 1952 off Lowestoft on the Hull-Rotterdam route. 
So what exactly was a danlayer? For every channel that was swept clear of mines by the sweepers, it had to be marked. They were marked by dan buoys, which, according to dictionary definitions, is specifically British English usage. A dan buoy is a ‘small buoy used as a marker at sea’, particularly ‘used as a marker in deep-sea fishing’ or ‘showing the limits of area cleared by minesweepers’, and with a ‘coded flag’ at the top.  So the danlayers followed the sweepers as the gulls follow a trawler at sea.
The photograph below shows a danlayer at work in clearance operations in the immediate post-war period.
Of the known danlayer losses of the Second World War within England’s Territorial Sea two have been discovered and charted, HMS Sargasso and HMS Elk (the remains of the latter not confirmed). HMS Sargasso has been identified off the coast of Dorset with weights for dan markers discovered aboard and forming part of the identification process. 
Already a rare group of vessels, confined to the war years, few have been identified, and only on this one example is the function of the vessel legible in the remains: the Sargasso has a significance beyond her size.
 Ministry of Information, 1943 His Majesty’s Minesweepers (London: HMSO)
 Historic England Research Records
 Historic England Research Records
 Entries for dan in this sense: Collins English Dictionary which notes that this is specifically British English; Concise Oxford Dictionary, 7th Ed., 1982 [Oxford: Clarendon Press]; not in Merriam-Webster [US English]; Danlayer
 UKHO 61787