A Sea Lord class concept could be themed with Azshara and using Arcane powers of the Old Gods. Thunderstorms and Lightning attacks would be derived from a dark, primal source of power. This class is a spin on an 'Arcane Archer' archetype, based on the Sea Witch and flavoured with Azshara and the 'Dark Below' expansion themes.
They could have a Ranged DPS spec using Bows, an Absorb-based Healer spec based on Mana Shield, and a Caster spec based on using Arcane and Storm powers. They would wear Mail armor. A Naga race could be introduced to usher in this class.
The use of Arcane and Storm powers (derived from Old Gods/Maelstrom/Azshara) by the Sea Lord is no different than Warlocks using Fire and Shadow, Mages using Arcane, Fire and Frost and Shamans using Fire, Frost and Storm. Each class has a distinct source for their magic.
The inclusion of playable Naga would help the separate Sea Lord class from Shamans, considering Naga do not use Shamanistic magic. This would open up the use of Storm powers to races without Shamans, such as Gnomes, Forsaken, Humans, Blood Elves and Night Elves.