Some people advise others to wash the shroud with Zam Zam water in order to seek blessings from it, is this the Sunnah, and if there is no evil in it, it is considered a misguided innovation?

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger. We ask Allaah to exalt his mention as well as that of his family and all his companions.

There is no evidence about this act in religion, and no scholar from our Salaf or the four Imaams did so, therefore, it is more appropriate to abandon this act.

As regards the other issue which you raised in your question, that an innovation is not an innovation unless it is harmful or evil, then this is not correct as no scholar has ever mentioned this to be a condition [for an act to be considered an innovation].

Allaah Knows best.