This paper presents a nonlinear, backstepping depth and pitch controller for a dual-bladder buoyancy engine actuated by gear pumps. Flow-rate feedback is obtained using a custom flow sensor comprised of a differential pressure sensor and a small, 3D-printed attachment. The controller is simulated using a model of the CephaloBot, our in-house developed autonomous underwater vehicle (AUV). Its depth control capability is also experimentally validated using a single-bladder buoyancy engine on-board a smaller-scale test cylinder. Lyapunov stability analysis shows global, asymptotic stability, which is exhibited in our simulation. Our experiments verify that this buoyancy engine is a feasible and effective depth controller for AUVs.