From Fallen London Wiki

word of advice for new players who may not already have one; the A Faceted Decanter of Drownie Effluvia is much more useful then just the +5 shadowy. if you have the effluvia and at least a small amount of KT (i forget the number; but i can 100% every time and have been able to for a while) you can get a very reliable troubled waters decrease while zailing. if you dont have one yet i would defo be aiming for it :D