Ideally, you’d book the flight tickets AFTER the charter agreement has been signed.
The reason is simple- it’s much easier to reschedule a flight for 12 people (even individually) than to get the boat you want, from the port you need, on the dates you have.
Flexibility with dates/locations can offer significant price savings compared to variants where boats have to be delivered.
The least favorable combination is last minute charters (1-4 months before the charter) with fixed dates, where the number of available boats is limited and the dates might not fit individual boat’s schedule (not all charters are Saturday – Saturday).