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In the well known book "South, The Endurance Expedition" by Sir Ernest 
Shackleton, in Appendix I, Physics the following is found

"During the voyage of the Endurance about fifteen months elapsed during which 
no check on the chronometers could be obtained by the observation of known 
land, and had no other check been applied there would have been the 
probability of large errors in the longitudes.  For the purpse of checking 
the chronometers a number of observations of occultations were observed 
during the winter of 1915.  An occultation is really the eclipse of a star by 
the moon.  A number of such eclipses occur monthly and are tabulated in the 
"Nautical Almanac"  From the data given there it is possible to compute the 
Greenwich time at which the phenomenom ought to occur for an observer 
situated at any place on the earth, provided his position is known within a 
few miles, which will always be the case.  The time of the disappearance of 
the start by the chronometer to be corrected is noted.  The actual Greenwich 
time of the occurance is calculated and the error of the chronometer is thus 
determined.  With ordinary care the chronometer error can be determined this 
way to within a few seconds, which is accurate for the purposes of 
navigation.  The principal difficulties of this method lie in the fact that 
comparatively few occultations occur, and those which do occur are usually 
stars of the fifth magnitude or lower.  In the Antarctic, conditions for 
observing occultation are rather favorable during the winter, since fifth 
magnitude stars can be seen with a small telescope at any time during the 
twenty four hours if the sky is clear and the moon is also often above the 
horizon for a large fraction of the time.  In the summer, however, the method 
is quite impossible, since, for some months, stars are not to be seen.  No 
chronometer check could be applied until June 1915.  On June 24 a series of 
four occultations were observed and the results of the obser vations showed 
an error in longitude of a whole degee.  In July, August and September 
further occultations were observed, and a fairly reliable rate was worked out 
for the chronometers and watches.  After the crushing of the ship on October 
27, 1915, no further occultations were observed, buth e calculated rates for 
the watches were employed and in the longitude deduced, using these rates on 
March 23, 1916, was only about 10' of arc in error, judging by the 
observations of Joinville land made on that day."

The celestial reductions, unfortunately, are not given in the book.  However, 
I think we can trust the word of Sir Ernest Shackleton!  Here is a clear case 
of occultations in use at sea.  While his use of the small telescope may 
bring the claim that the occulatation was not at sea, the short answer is 
that the Endurance was trapped in the ice during this time.  The position 
within the Weddell Sea, albeit it frozen in the pack ice, still qualifies as 
"at sea"!

Best Regards
Brad 










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