We did a shared water taxi into Venice, and we did not book ahead. We decided to just scope it all out when we landed. We found a water taxi stand within the airport, inquired, paid (much less expensive than booking a private one for just the two of us). We waited until the coordinator had a full load (I think it was a total of 7 of us), and then we walked to the dock and took off. I think I recall it was about 45 minutes total until we went off as a group. It was fun chatting a bit with the other passengers, two from Canada and three from Germany.
The experience was incredible, and truly a memory we will have for life. The look on my husband's face as we approached Venice was priceless, then going up the little canals (driver warning us to duck when we went under some bridges) was so lovely and unique.
We were so thrilled we chose the water taxi over the water bus thing, which would not have offered the same views.
We later took the Vaparetto (sp?) on the main loop of Venice (as was suggested in the Rick Steves' guide), also waiting to catch a seat in front of the wheelhouse (as his guidebook suggests), and it was also a fun experience where we saw a lot of Venice from a different view.