Beach diving for me the number is one. In the water it's hard to talk so you have to concentrate on diving. Two divers can cooperate well enough to be able to do pretty much anything that can come up. But I haven't really done three people too often so maybe it's good too. When I do go beach diving I usually go alone.
Boat diving I'm undecided between 2 or 3. Three people total is logically better, it's all that's needed if the boat needs to be driven there's never less than a pair of divers in the water. However 4 feels more like a party. 5 is doable and can be nice, but it can also be a mess. Any more is too much for my taste.
What's your take? I'm expecting some varying opinions.