This letter investigates the model-based control of a class of soft continuum manipulators with hydraulic actuation that bend on a plane due to pressurization of one or more internal chambers. A port-Hamiltonian formulation is employed to describe the system dynamics, which includes the pressure dynamics of the hydraulic fluid. A new nonlinear control law is constructed with an energy-shaping approach, and it is combined with an adaptive observer to compensate the effect of unknown external forces. Stability conditions are investigated with a Lyapunov approach, and the effect of the tuning parameters and of key model parameters is discussed. The effectiveness of the controller is demonstrated with numerical simulations.