You could also take a vaporetto there, and walk back if you prefer. A one-way vaporetto ride is 6.50 euros per person. You want vaporetto #1 as that one goes a little faster. You would board at Piazzale Roma, which is only a peoplemover (monorail type) ride away for 1 euro. An all day vaporetto pass is 16 euros if you really want to limit walking. The Doge's Palace is the really decorated pink building in Piazza San Marco, and the visiter's entrance is closest to the water. It is amazing, but buy a guide book or rent an audio guide as there is so much interesting history and art here.
:Cruise Blog/Website with Review of Carnival Magic