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."
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Sea Princess 2014
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
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.