As a rule, propellors on multi-propellor[ u]seagoing[/u] vessels turnoutward when moving the ship forward. Having the propellors turn outward prevents the hydrodynamic forces created by them from reinforcing each other, which might cause damage to the ship and/or the props (cavitation for example).
But why should they turn in opposite directions? Well, a turning propellor, as we all know, causes momentum, affecting the ship 's course unless this is corrected with the rudder. (Sailing a one-propped model backwards shows this effect most clearly).
This effect would be much stronger and possibly even impossible to correct with all propellors turning in the same direction. With the propellors turning in opposite directions, they will compensate each other´s momentum and the ship will keep a straight(er) course.
On non-seagoing multi-propped vessels, the propellors still turn in opposite directions, but this time they turn inward when moving the vessel forward. I'm not sure, but I'm guessing this has to do with the different hydrodynamic conditions of shallower and/or sweet water.