How to find an address in venice?

I am trying to find an address in Venice. All I have is a number and island name - which doesn't correspond to any of the online maps I have searched - IE Google, Multimap etc. I don't have a street name, though all the online maps do. Is this normal? Have I just been given insufficient info or if not how do I locate pinpoint this on a map? The address is 489 Giudecca.