Would the mount created by the Find steed spell be effected by the water walking spell?

1/2 Would the mount created by the Find steed spell be able to be effected by the water walking spell? My AL dm argued no as.."10 willing creatures you can see gain this ability for the duration." his point is the mount created isn't a creature but that seems silly

— c (@rawrzmoo) July 18, 2017

should work fine – it's a creature

— Mike Mearls (@mikemearls) July 18, 2017