The reason to take the whole yacht is not the price per head — it is that the boat stops where you want it to.
What do you get for taking the whole yacht?
Four hours with twelve guests is €680 plus €180 for the four extra places, €860 in total, and the yacht is yours for those four hours: the swim stop lasts as long as your group wants, the paddle boards are not queued for, and the playlist is not a stranger's.
How many people make the private yacht worth it?
Eight people on the shared departure pay €633.68 between them, against €680 for the private boat — about €46 for the difference between sitting with strangers and not. Below six, the shared trip is clearly cheaper; from seven or eight upward, the gap closes to almost nothing.
What should you settle before you book?
Pickup and return are included when you book in advance, so give the name of your hotel while booking rather than on the morning of the trip.
Important
the watersports are billed separately at fixed rates, so a group that plans to jet ski or parasail should add that to the budget before comparing the two formats.