For four people the difference is about €230. By nine guests it has vanished. Here is where the line actually falls.
The arithmetic
Our group half day is €55 per person. A private half day is €450 for the whole boat. So two people pay €110 shared against €450 private; six pay €330 against €450; and at eleven the private boat works out at about €41 a head — cheaper than the shared tour. The crossover sits at nine guests.
For full days there is no shared option: €600 for the boat on any of the three routes, all seven hours. At eleven guests that is about €55 a head; at two it is €300.
What money actually buys
Control, mostly. On a private charter you set the departure time, change the route mid-day, stay two hours at a cove you like and skip one you do not. If someone wants to swim for an hour while everyone else eats, that happens. Nothing is on a clock.
On the shared tour the route is fixed and the timings are fixed. What you do get — and this is the part people do not expect — is a maximum of eleven passengers, which is a completely different experience from the sixty- and eighty-seat boats that dominate this coast.
How we would choose
Couples and solo travellers: the group tour, without hesitation. Families with small children: private, because flexibility around naps and food is worth more than the money. Groups of six or more: private, because you are close to paying for it anyway. Anyone with one free day and a specific idea of how they want to spend it: the custom day.