There's all kinds of research on how to make something cut through the water with minimal resistance, but what if resistance was exactly what you wanted? What kinds of shapes create above-average resistance with water? Also, does the hydrophobicity of a material affect its fluid-resistance in water? Because hydrophobic materials are pretty jagged...but they also repel water. Note that this is different than drag as drag deals with aerodynamics.
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.