I don't know, it just doesn't seem to fit the theme of the concept, for me. I feel it's like the same as giving a tank spec to Priests, because they have shields. In my mind, for a class to have a tank spec, there should be intense physical training in the class' background, like it happens for Druids, Monks, Warriors, Paladins and Death Knights, and I just can't see any sort of 'intense physical training' in the Sea Witch's background.
Sure, the 'mana shield tanking' idea may sound interesting, but maybe such a mechanic would be better off being given to another class that has actual physical training in its background?