Just me or does the Resto Shaman build seem rather...dry?

For Chain Heal, a convenient, math-free implementation could be:

Roll a number of dice equal to rage damage bonus.  Line them up and sort from high to low.

Heal the primary target for the total (this could include your modifier), heal a second target in range of the first for the total omitting the lowest die, heal a third for the total omitting the lowest two dice, etc.  

Concerning the (lack of) limiting resource problem, it seems like one could imagine that using soothing water and healing is not channeling rage, but competing against it, such a Resto shaman might need to alternate between generating rage through damage or other resource management, and spending rage to heal.  Or (equivalently) they enter rage with a Water Shield containing a certain number of charges, which eventually requires replenishment as they "dry out" from healing.

Quote from Elioron >>

I added being in combat to the conditions for Healing Rain - "While your Rage is active and you are in combat..." It simplifies it while eliminating the out-of-combat use.

The problem is, there's no rules definition of what "in combat" means. If enemies are approaching and the DM gives you a turn to prepare so you activate your Rage, are you "in combat" yet? Does combat end the moment the last enemy falls? What if it's a scenario with a lot of quick skirmishes, and a lot of movement in between -- are you "in combat" while moving between the skirmishes? 

Panta's right, making it a bonus action and not a Magic action would be the cleanest way to accomplish what you want