We went to Venice today and first off, bring lots of room on your camera device. Venice is so picturesque you will be snapping every which way you turn. It's that gorgeous.
Get the water bus pass, especially if you're staying overnight (get the the 12 hour one). Venice is a walking city and a boating city. But a single afternoon is not really enough time to see much of the insides of places (museums, cathedrals). The city is very crowded (see the picture of the water bus). This is only chilly January and we saw crowds everywhere, I can only imagine what it's like in high season.
The streets are not so well marked and what I saw on Tripadvisor was that people had a hard time finding there hotel/hostel. If you can arrive to Venice during the light of day, you'll be in better shape for finding your accommodation than if you roll in at 10pm. Dark alleys are everywhere. During the brilliant light of day, they are charming and romantic. In the chill of night, those alleys can feel creepy and foreboding.