With travel schedules being what they are I was considering the "open ticket" option with Vedettes du Pont Neuf, they say just show up and you'll get on the next boat. But I'm ASSUMING, if they've sold out of people who specifically booked that day and time, you're out of luck. What's been people's experience with that?
Vedettes du Pont Neuf also says "subject to availability". I don't know if they set aside a certain number of places on each cruise for open tickets. But, open tickets are the last to board, so you're not guaranteed seating on the upper deck.
The cruises can sell out, especially if there is a large group booking or an improvement in the weather after a wet spell.
Based on my experience last April, I was told by the ticket office if you purchase a timed ticket you can always use it on a later time/day as a space available ticket.