I can't help you , but I'll be watching for experienced cruiser's answers! We'd love to fly to Europe and cruise back, but the cost of flying is more than the cost of the cruise ! One way flight's are ridiculously priced, IMHO.
"A truly happy person is one who can enjoy the scenery on a detour."
Golden Princess 2009 Carnival Conquest 2010
Grand Princess 2010 Island Princess 2011
Grand Princess 2011 Carnival Magic 2012
Carnival Dream 2012 Island Princess 2013
Carnival Magic 2013 Carnival Legend 2014
Carnival Magic 2015 Coral Princess 2015
If you are happy to change in Europe, then both Amsterdam (Schiphol) and London (Heathrow and Gatwick) have several flights a day from Venice, and then on to Houston. This includes budget airlines such as EasyJet, RyanAir or Transavia, KLM, and BA (If they are not on strike).
So look for a few routes, then shop around on line options.
Alan & Katrina
Last Cruise: Queen Victoria November 2014
Next: P&O Britannia March 2015
Then: Minerva April 2015
and : Royal Princess August 2015
I think this is the one situation I would check and see what the Cruise's air fare is.. They may be able to get certain breaks for a one way ticket...Be sure to check with the airline too and see if there is a difference.