I live in Venezia and can tell you that you don't need to prebook. They are everywhere and the prices are set so when they say special price...okay special as in the same price for everyone. It's their thing, trying to draw people in. Trust me, they are not going to rip you off. They are all the same price IF you are doing the same route. So don't bother to dicker with them.
It's not cheesy at all....it is a great way to see Venezia. Just know that you will be mostly in side canals and only briefly on the grand canal....it is SO busy with vaporettos, taxis, delivery boats, polizia, trash boats...you will be happy to get into the back canals.
Enjoy!