This really does depend on when the flight is, how much luggage you expect to have, whether you are willing to change airlines en-route, and whether you are looking for direct flights.
Take a look at the airport website (http://www.europe-airports.com/italy...?code=VCE) for
ideas of route oprions AA fly Venice to Madrid & onto the USA, whilst UA fly via Frankfurt
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: Cunard Queen Victoria January 2016
Next: Oceania Nautica Aug 2016
Then: P&O Oceana December 2016
Golden, Grand(2), Caribbean, Royal, Sea(4+1 aborted)
Adonia(4+2), Arcadia, Artemis(2), Aurora(5), Azura(2), Britannia, Oceana(3), Oriana(12), Ventura(2), Victoria(5)
QM2(2), QE2(9), QV(7), QE, Caronia(2), Minerva, Minerva II, Fred.Olsen(8), NCL Jade, =X= Eclipse, HAL Ryndam, Aza.Journey