Even though ALL of the literature says that things like sweetflag and blueflag and arrowhead can be planted in 0-6" water over the crown, do you feel that they do BETTER in only 2"-3" - i tend to think that they are challenged with rot and water logged foliage at 5-6" even though they are 'rated' for 6" in the case of many marginals* as above.

In my ponds, the sweetflag does well. I think the deepest I have it is 4". The blue iris and arrowhead don't do as well (they're alive but don't really grow much). They are both in less than 3". So, I do think that while some plants may survive at 6" deep, they will do much better in less than 3".