Diving Masks come from a special capped Mask Locker that restocks automatically over time.
You can earn Masks by winning PvP duels, Clan War assaults and skirmishes, and taking part in Tournament, as long as there are Masks left in the Locker. By default you get 1 Mask restocked in the Locker per 1 hour. Atlantis Permits increase maximum Locker capacity, restocking speed, and the number Masks you receive per PvP win.
If you don't receive Masks after the won battle - most likely there are no Masks available in the Locker.