I run into this issue also, but I find that having the group self targeting lesser healing wave is usually the best way to go. I also have macros for each shaman to cast any heal on one target at the time if needed, but this is usually for if a target is getting focused.

Anyways, I believe the self casting lesser healing wave is the most efficient healing and mana wise in most cases.

Maybe some one knows different?

BTW, this is speaking from a PvP standpoint, not sure what situation you are in. Plus, I like the above post too for when you need that Chain Heal. =)