I have activated mount equipment for water walking on a mount and it works on my 120 , it does not work on my level 37 shaman or my 70+ priest.
Mount slots are per-character, not account-wide, so unless you bought and distributed waterwalking items to your alts, they don’t get waterwalking.
(source: question and answer from comments to this Blizz announcement.)
Ouch!
Thank you for the heads up.