Shouldn't this work for OP? I know it's from a long time ago... but I was looking through Druid macros and came across this.
Code:
#showtooltip
/stopmacro [mounted]
/cast [swimming]Aquatic Form;[nocombat, flyable]Flying Mount Name;[nocombat]Ground Mount Name;[combat]Travel Form
/cast [nocombat, flyable]Swift Flight Form;[nocombat]Travel Form
Only downside I can forsee is no aquatic mode if mounted, and can't switch from ground mount to flying mount.
Code:
#showtooltip
/cast [swimming]Aquatic Form;[nocombat, flyable]Flying Mount Name
/stopmacro [mounted]
/cast [nocombat]Ground Mount Name;[combat]Travel Form
/cast [nocombat, flyable]Swift Flight Form;[nocombat]Travel Form
This "should" always allow Aquatic Form, and also allow switch from ground mount to flying mount.
I don't have a high enough druid to test this on, but it seems like it should work?