Hi Bryoung
I had some credits on 1837online and looked up the deaths at sea from 1865 - 1871. Unfortunately there were only 4 Youngs - initials C, J, N and Mrs Young, so not wee Elizabeth and Helena.
1837online has births, marriages and deaths of passengers at sea "including British subjects and other nationalities". I'm not at all sure if that would include Scottish people
There are likely other sources, but as my credits expire on 1 June I thought it was worth a try.
At least we can rule out 1837online as a possible source.
Incidentally if anyone reading this could use a lookup in the next day or so, let me know - the credits are useless to me.
Best wishes
Pinkshoes
later : The Family Record Centre apparently holds Scottish BMDs at Sea but might involve a visit?
http://www.familyrecords.gov.uk/frc/res ... datsea.htm
JJ