Minimum depth is a bit dicey - in that it all depend on which species you dealing with.
Zoodiver's guideline is certainly ideal for most sharks. And I would say it's such be a requirement when keeping active swimming sharks such as Smoothhounds, Carcharhinids and Hammerheads.
However benthic sharks are a bit different. As most benthic sharks rarely go more than a few feet from tin the wild, even when they are in water that is more than 33 feet(10 meters) deep.
The absolute bare minimum tank depth - I would consider for any benthic shark would be about 18" . Even then that would only be for those species which are native to tidal pools - such as Coral Catsharks, & Epaulettes.
For bamboos and small wobbegongs - the minimum depth should be 24".
For adult Heterodontids(Horn sharks), I would go with at least 30-36" depth. As horn sharks - tend to of a stockier build and they tend to be a bit more active.
For an adult Nurse Shark - at least 4 feet deep - although depth of 6 ft or more would be ideal. These are massive sharks for a private aquarist - and really require a tank/pond fit for a small to medium-sized(4-6') requiem shark.