Greg responds...

Garth's specialty is summoning the Tempest, i.e storm magic.

Tula's abilities are more or less the same as Kaldur's except she's more advanced so doesn't require the Water-Bearers that he uses.

Mera is a master sorceress, who doesn't need to specialize as much. But she can create solid forms out of water like the others, but has more flexibility, imbuing her creations with something akin to a life of their own.

Zatanna's abilities are even less specific. She can accomplish many, many things with enough preparation and energy.