If you'll forgive a slight expansion from fighter pilots over the ocean to aircrew over water more generally: firstly, signalling position. In the air a pilot had two main methods: sending an SOS message over the radio, and once Identification Friend or Foe (IFF) transponders were introduced in 1940 these had an Emergency or Distress setting that would show up on friendly radar screens if within range (initially this would only be land-based radar, airborne radar was later introduced that could also interrogate IFF). The "Bailing Out Over Water" section of Army Manual AAF 51-127-5, Pilot training Manual for the P-51 Mustang, gives a typical procedure:
"1. Notify wingmen that your airplane is in trouble.
2. If your airplane is equipped with IFF, turn the emergency switch ON.
3. Transmit "Mayday" three times, followed by the call sign of your plane three times.
4. Your first transmission will be on the assigned air-ground frequency. If you cannot establish communication on this frequency, use any other available frequency in an effort to establish contact with a ground station. "
Once in the water, larger aircraft with dinghies might have an [emergency manually powered radio] (http://www.tangmere-museum.org.uk/artefact-month/the-museums-gibson-girl-transmitter), Type T.1333 in the RAF, SCR-578 in the USAAF (or more commonly "Gibson Girl" from its hourglass shape). To signal a friendly aircraft crew could use mirrors, a dye marker to stain the water a vivid colour, or a Very pistol to fire a flare. Some RAF aircraft also had an additional method of sending a distress signal: a pigeon. One or more pigeons could be carried on the aircraft in [a basket] (http://www.iwm.org.uk/collections/item/object/30103738) and released with a message after ditching. "[The use of Pigeons by the RAF] (http://www.arcre.com/archive/pigeons/pigeonsraf)" lists several examples, some birds received the [Dickin Medal] (https://www.pdsa.org.uk/what-we-do/animal-honours/the-dickin-medal) for their part in rescue operations like [Winkie] (http://www.bbc.co.uk/news/uk-scotland-tayside-central-17138990).
Chances of survival varied greatly. During the Battle of Britain, for an RAF pilot it was minimal. The English Channel is hardly the most daunting body of water, but even in the summer a downed pilot could only survive for about four hours, and the RAF had made almost no preparation for the eventuality. German pilots had a yellow life jacket, inflated by CO2 bottle, with fluorescent dye to mark their location. The standard early RAF 'Mae West' life jacket, by contrast, had to be manually inflated (not easy after landing by parachute or escaping from a ditched aircraft, especially if wounded), was dark green (pretty much the same colour as the Channel), and the pilot had no means of indicating his position such as marker dye. To make matters worse, though the Luftwaffe had instituted a Seenotdienst (sea rescue service) in 1935 and operated both boats and He 59 seaplanes equipped for rescue operations during the Battle, bizarrely the RAF had no equivalent. As if that all wasn't bad enough, the collar of the standard issue officer's shirt shrank in contact with sea water, making even breathing more of a challenge. During July, when much of the fighting was over the Channel, 220 aircrew were posted as missing or killed, the majority lost over the sea.
Matters greatly improved over the course of the war, with improved equipment issued to aircrew and a Directorate of Air/Sea Rescue established in February 1941. This brought dedicated ASR squadrons that generally flew hand-me-down aircraft no longer required for other duties, such as the Westland Lysander and Boulton Paul Defiant, which would search for survivors floating in the water, drop survival equipment such as a dinghy, mark the location with a smoke bomb and summon either a boat or seaplane such as the Supermarine Walrus (affectionately known as the Shagbat). The dedicated squadrons could also be joined by impromptu search missions, especially from comrades of missing crew.
By 1943 there were eight ASR squadrons around the UK; those that operated in potentially contested areas such as the Channel flew [old Spitfires with minor modifications to drop survival packs and smoke bombs] (http://www.iwm.org.uk/collections/item/object/205210423), but still armed; 277 Squadron even shot down five V-1 flying bombs in 1944. For longer range operations there were Lockheed Hudsons and [Avro Ansons] (http://www.iwm.org.uk/collections/item/object/205210347), and from late 1943 Vickers Warwicks, originally designed as bombers but not used in that role; the Warwick had long range, good endurance, ASV radar, several pairs of eyes for spotting, and rather than inflatable dinghies it could carry and drop by parachute the Mk IA Airborne Lifeboat, a 10m wooden boat with engines. Very long range rescues could also be effected by the large Sunderland and Catalina flying boats of Coastal Command; in July 1944, a Catalina of 210 Squadron rescued six survivors from a Liberator who had been in the sea for two days, 540 miles north of the Shetlands. Chances of survival were much better, but still not a certainty by any means; in 1943 1,188 aircraft were lost at sea, mostly bombers, with 4,466 aircrew; 1,684 were rescued.
I'm afraid I'm not so familiar with overseas operations and other air forces; the US Navy, for example, deployed submarines on the route of long-range B-29 missions to be able to rescue crew who had to ditch, but I don't have statistics to hand on numbers of losses or rescues.
Main sources:
Air Cdre Graham Pitchfork, "The Evolution of the Air/Sea Rescue Organisation", RAF Historical Society Journal #40
The Army Air Forces in World War II, Vol. VII: Services Around the World, Chapter 15: Air-Sea Rescue
Andy Saunders, Convoy Peewit
Stephen Bungay, Most Dangerous Enemy