You could use a roll of a 20 sided dice to determine that, just like in the DnD tabletop games, with 20 being absolute success and 1 being absolute failure. Nobody can check it, but as you said it's an honor system only
As for how to interpret the results of each roll of the dice:
20 (absolute success): a friendly seaplane/rescue boat picks you up immediately after the sortie is done, you are back at home base a couple of hours later, barely inconvenienced and not shaken at all
15-19: you paddle around in your dinghy for 10 hours before you spot some movement, try to make out their markings and finally deciding to fire off a flare, lucky you, it's a friendly ship and you get back to base, tired and soaked in cold water, but safe and sound nevertheless
10-15: you paddle around in your dinghy for 10 hours before you spot a ship coming your way, unfortunately it's an enemy trawler and the war for you is over, you are captured
2-9: you don't even know how long you've been out here at this point, you've lost track of time completely and your rations and clean water are almost finished, pilot is never found and presumed dead from exposure to the elements
1 (complete failure): your dinghy is already shot up and taking water when you abandon your cockpit, you are forced to swim it out in your mae west while enemy aircraft and vessels are firing at your direction, a pack of migratory sharks appears and eats your pilot
