I agree with the response above that there is no workable theory for the drag coefficient of an arbitrary shape. However, I feel like I might can shed light on the "least efficient" shape to move through water.
Based on drag being the result of momentum conservation, my guess is that a flat plate moving "bulldozer" style through water is about as much resistance as I can think of offhand. If I were trying to have some form of effective braking system for a watercraft, the flat-plate approach would be my starting point.
The larger and more rigid the plate, the better of course.
With regard to the hydrophobicity part of your question: I've never calculated this (and don't know how to, frankly), but I strongly suspect that the hydrophobicity or hydrophilicity of the material plays an ignorable role in the drag. Actual physical features of the object surface might play a role, though. For example, is the surface smooth, or pelleted like a golf-ball. I'm sure the golf ball type surface would have more drag.