Take a regular taxi, not a water taxi in Venice; relatively fast and cheap. Rome I would either arrange for a car/limo or take taxi to airport. Long trip but fast. Real question is why you are not doing a pre-cruise in Venice and/or post-cruise in Rome; both fantastic cities.
"Carpe diem quam minimum credula postero"
Seven Seas Explorer - Miami - Venice - Mar 2017
Silverseas Explorer - Arctic - Aug 2017
Seven Seas Voyager - Rome to Beijing - Oct 2017
Seven Seas Mariner - Lima to Buenos Aires - Jan 2018
Silverseas Explorer - Siberia - Jul 2018
Seven Seas Mariner - Australia Circumnavigation - Dec 2018