BJ:
You may want to contact your cruise company.
A- Because you'll need them to do the invite letter anyway
B- Because they may be able to arrange a short term visa for you.
See
http://www.waytorussia.net/RussianVisa/Types.html
(Scroll down to Short 72-Hour Visas)
Short 72-Hour Visas:
This type of visa WAS an experiment started on the 1st February, 2002 to attract more tourists to Russia. However, there are canceled as of now, so it is NOT possible to get a 72 hour visa at the moment, it's better to go for a tourist visa.
However, if you are traveling on a cruise ship, the cruise company should be able to provide you with a short visa, usually valid for up to 72 hours.