We arrived un Venice on Saturday morning on an overnight flight from Philadelphia. There were four of us. I did not reserve a private water taxi in advance as folks here on CC suggested it wasn't necessary. When we arrived and cleared customs we went to the water taxi desk in the arrivals hall. There are about four companies at the desk. Only one could accommodate us. When the first company (motoscafi Venezia) told me they could only accommodate pre-registered guests I became very nervous.
It all worked out but it would have been better to reserve in advance.
BTW I highly recommend this method of transfer to your hotel. It took about 30 to 40 minutes to get to the Molino Stucky Hilton but we meandered through many canals including the grand and what a way to enter city. Cost was 120 € for four people and eight bags