I agree with your estimate. The man who navigated an open lifeboat the farthest distance is Capt William Bligh, RN and former Commanding Officer of the HMS BOUNTY. The BOUNTY was the third ship under his command to have mutinied against him. The BOUNTY was also the last ship he was ever allowed to command.

Navigators in those days had to know their stuff and I imagine the navigator on Shackleton's expedition would have been a cut above excellent, his bio (see the wiki link in AKSAR's post) seems to confirm. I wonder how precise his sextant was capable of measuring... Point being that he used celestial to navigate close enough to get a visual on South Georgia. It doesn't really matter if they were to the left, right or on centerline; they were close enough to get a visual "through mists and squalls" -- close enough. Helluva feat.

On an anecdotal track, Robert Grant, in "Bush Flying: The Romance of the North" relates a story about celestial navigation saving his hide. He was getting some inflight celestial navigation practice, and his reading contradicted some piece of cockpit nav gear (IIRC, it was an ADF that the pilot was relying on). As it turned out, the ADF was malfunctioning and the aircraft was very off course, and headed out over the ocean.

IMO, Celestial Navigation is like reading Morse code or Semaphores. You may not ever have to use it, but when you need it, you really need it. It is also a relatively fail proof answer to the universal "Where the **** am I!?!" question that seems to indicate needing a rescue.