Large bottles in your case could break and soak the contents. Most cruise lines allow you to bring on 2 bottles of wine when you have been ashore, so this might be a better idea, but don't forget the cork screw !
We were on the carnival glory in May 2008 in St Vincents virgin island and brought 2 bottles of wine aboard, but these were confiscated as we boarded and were given back on the last night of the cruise so beware!!!!!! Edd.
Your resourcefulness to save a few pounds is examplery - what about jereboams? Not getting off the ship at St Petersburg will also save a few more pounds.
The technical answer to this question is that a "bottle" is usually defined as 75cl. I am sure that this is the size that Celebrity were thinking of when they made this rule. Try the larger size if you are really that desperate. You only risk loosing a few quids worth of cheap wine if they are confiscated after all! Happy cruising.