Either go:

/castsequence [nomod, target=player] Water Breathing, Water Walking; [mod:alt, target=Toon2] Water Breathing, Water Walking; [mod:ctrl, target=Toon3] etc.....

Or have the normal castsequence without using modifiers to specify targets, and just manually choose the shammies target and click the macro.
Then change to the next target and click the macro.. this approach lets you cast on others, not just those defined in the macro.

You could even have....

Background Shammy Toon...
/assist MainToon
/castsequence Water Walking, Water Breathing