Tide change in 4.2 was primarily aimed at reducing

The Mana Tide change in 4.2 was primarily aimed at reducing the mana available to non-shaman healers in raids. In short, shaman are balanced around always having their own Mana Tide, but other healers are instead balanced around their personal cooldowns (Shadowfiend, Innervate, etc.). Adding Tide on top of those often led to other healers having access to so much mana that managing the resource became a non-issue in many situations. The change from Improved Water Shield to Resurgence was designed to offset the Mana Tide reduction’s personal impact to the shaman.