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.
"The test of a first rate intelligence is the ability to hold two opposed ideas in the mind at the same time and still retain the ability to function."
F Scott Fitzgerald
Silversea Silver Explorer (23nts) - Kangerlussuaq, Greenland - Nome, Alaska - Aug 14
Seven Seas Voyager (30nts) - Dubai - Cape Town - Nov 14