Atlantic Ocean

The crew (228th Squadron) was engaged in an anti submarine patrol mission over the Bay of Biscay, and left RAF Pembroke Dock at 0537LT. The seaplane aircraft was flying in the clouds and when it came out of the cloudy area, it was spotted by the pilot of a Luftwaffe Focke-Wulf Fw.190. It was shot down and crashed some 190 km west of Brest, France. All 11 occupants were killed.

The aircraft left RAF Lyneham at 2230LT on a transit flight to Gibraltar but failed to arrive. No trace of the aircraft nor the crew was found and it is believed it crashed into the Atlantic Ocean or the Bay of Biscay.
Crew (512th Squadron):
W/Cdr Patrick E. Hadow,
F/O R. L. Hammill,
LAC Charles W. Summers,
W/O Cyril Frederick Dean Steel.

The seaplane left RAF Pembroke Dock at 2345LT on December 18 on a routine flight to Gibraltar with 19 crew members on board (two complete crews and reinforcements to the squadron as passengers). While cruising by night some 30 miles off the Saint-Vincent Cape located on the southwest coast of Portugal, the crew was able to send a distress IFF signal that was received by the crew of a second Short Sunderland which was following DW106. Despite intense SAR operations, no trace of the aircraft nor the crew was found.
Crew (270th Squadron):
F/O Vernon Maurice Sparkes,
F/Sgt Charles Albert Mark Barber,
F/Lt John Eric Wood,
F/Sgt Frederic Raymond Brown,
F/O Edward James Jay,
F/Sgt Francis Mcphee,
F/O Reginald Robert Terance Crump,
F/Sgt Eric Piggott Botting,
P/O Neil Procter Chapman,
F/O Norman Edmond Hanna,
Sgt Stanley Kidd,
F/O Raymond John Elderfield,
Sgt William Henry Cryer,
Sgt William Peter Houston,
Sgt Henry Leonard Thompson,
Sgt Maurice Robert Hunt,
Sgt Edmund Albert Hooker,
Sgt Samuel Hughes,
F/Sgt Leslie Robinson.
Source:
http://www.landinportugal.org/air_pages/a01_copy(133).htm

The float plane left Killadeas NAS, Northern Ireland, on a training mission over the Atlantic Ocean when he crashed in unknown circumstances into the ocean. No trace of the aircraft nor the 9 crew members was ever found.
Crew (131st OTU):
F/O Francis Peter Graves,
Sgt Robert Anderson,
Sgt Edwin Alfred Dennis Barnes,
Sgt Earl Darragh Morrison,
Sgt Alexander Fordyce Sutherland,
Sgt Allan Douglas Warder,
Sgt John Glynne Williams,
Sgt Owen Douglas Hodgkison,
Sgt John George Ley.

The crew left RAF Enniskillen at 1145LT on a bombing mission over the Atlantic Ocean. Following the attack of the German U-Boat registered U-648, the crew gained altitude and left the combat area. Approximately one hour later, the crew sent several mayday messages and informed ground that he was forced to attempt an emergency landing. The captain ditched the aircraft some 900 km west off Vigo, Spain, around 1745LT. SAR operations were deployed but no trace of the aircraft nor the 11 crew was ever found.
Crew:
F/Sgt Bruce Goulden Burton,
F/Sgt Robert Aird Park,
Sgt Norman Percival Cook,
Sgt Ronald Montague Fisher,
Sgt Noel Neil Lewis,
Sgt Walter McKay,
F/O Charles Gordon Gorrie,
F/O Wilfred Sydney Johnston,
F/O Robert Harry Strauss,
F/O Jan David Butler Ulrichsen,
F/Sgt Norman Barrett.

The seaplane left RAF Castle Archdale around 0500LT on a routine maritime patrol flight to Reykjavik, Iceland. Around 0915LT, the crew spotted the German U-Boat registered U-489 that was performing its first sortie from Kiel. The crew of the Sunderland completed four attacks when he was hit by enemy fire. The captain was forced to ditch the aircraft that sank within 5 minutes. Five crew members were killed while six others were rescued. The U-Boat was lost as well.
Crew (423rd Squadron):
F/O Harry Bertram Parliament,
Sgt Frank Ginger Hadcroft 9.

The crew was engaged in an anti-submarine patrol flight over the Atlantic Ocean. En route, the German submarine U-572 was spotted, cruising some 550 km off the Guyana coast. The crew of the Mariner attacked the submarine that was sunk. But it seems the Mariner was shot down as well and crashed into the sea at 11°35' N 54°06' W. No trace of the aircraft nor the crew was found.
Crew:
Lt Clifford C. Cox 11.

Crashed in unknown circumstances into the Atlantic Ocean while on a flight from RAF Castle Archdale to Gibraltar. No trace of the aircraft nor the crew was ever found.

The aircraft was involved in an escort mission. By night, the crew encountered technical problems with an engine and the captain decided to ditch the aircraft at 0030LT. Two crew members were killed while six others were rescued.
Crew (86th Squadron):
F/Sgt John Richard Fast,
Sgt Hollick.

The crew left RAF Hamworthy (Poole, Dorset) at 1900LT on a maritime patrol flight over the Bay of Biscay and was scheduled to return a day later around 1400LT. The seaplane failed to return and crashed into the Atlantic Ocean in unknown circumstances. No trace of the aircraft nor the 10 crew members was ever found.