In this letter, we consider the problem of regulating the voltage of an islanded Direct Current (DC) network subject to (i) unknown ZIP-loads, i.e., nonlinear loads with the parallel combination of constant impedance (Z), current (I) and power (P) components, and (ii) unknown time-varying disturbances. Using the port-Hamiltonian framework, two decentralized passivity-based control schemes are designed. It is shown that, using the proposed controllers, the desired equilibrium is exponentially stable and local input-to-state stable (LISS) with respect to unknown time-varying disturbances.