It seems to me that a Lead Core set up would be the best of all worlds for your application... then, you could run just about any crank you want, at the desired depth.
Also, don't forget about Offshore's snap weight system as another alternative.
For cranks, try; Wally Divers, LJ Power Dives, Manns Stretch 20, HotnTot, any deep diving Rap ...
HH