the VeniceCard is not good for a one day stay. May I recommend the One day Travel Card
for Euro 12. The card is good for 24 hours after stamping.
An adult standard one way ticket costs Euro 5,-- and is avlid for 60 minutes after stamping.
More information here
Do you know Venice? Here is a map
of the cty. On the left side you will find the cruise terminal (Stazione Marittima). From there some cruise lines have a shuttle bus to the Piazzale Roma. From there you can take the line 1 for the Canal Grande (aboput one hour from the Piazzale Roma to Piazza S. Marco.
Have a nice day in "my city" Venice