For beauty & photography, Grand Canal Vaporetto Ride (Line 1) delivers.
Line 1 chugs the full length of the Grand Canal past every palazzo facade for the price of a transit ticket — it's the single best-value 'tour' in the city, especially from a front rail seat at dusk. The catch: it's a working commuter boat, so it's packed in summer, you may stand the whole way, and a single ride is a steep €9.50 unless you buy a multi-day pass.