Why City of Venice is on water from the time it has been built?

Solomon is correct. You can find plenty of excellent information about the very early history of Venice and that area just by doing some searching on the internet. It is really extremely interesting!

i think that answer it's wrong...the luck of venice is that the lagoon it's in a very strategic positon in the mediterranean sea and it was since romans in the middle of the commercial traffics beetwen middle-east and northern europe. Venice at begininng can be seen like the join of many primitive settlements over the lagoon wich was composed by many little islands rich of fish food and other goods. Furthermore in the area of italy close to venezia there were many forrests. That's very important because below the buildings in venezia there are millions of wooden poles.