First of all, it depends on the crew. For each of the boats you will find icons like this one
indicating the composition of the crew best suited to this boat. For example: for 4 couples (8 adults), choose a boat with this icon
, this boat will have 4 double cabins. Then it will depend on the level of comfort/equipment desired: the boats are classified by stars: from 1 (old boat poorly equipped) to 5 (new boat with exceptional comfort and equipment).
NB: it is not necessary to take a boat bigger than necessary; if you are 2 couples, no use to take a 3 cabin boat, it is better to take a 2 cabin boat, but more comfortable.