Operating a multi-stage flash evaporation plant is usually a task of combining conflicting objectives. In this paper, the emphasis lies on maximizing the total fresh water production while maintaining good efficiency (ratio of produced distillate against total throughput of seawater) of the plant. Based on the evaluation of a cost function combined with motion-planning, an optimal operating point is obtained as a reference liquid level profile throughout the tanks of the plant. The plant has a cascade structure and the entire height profile in all stages is controlled by manipulating the flow rate into the first chamber. Nonlinear and linear control techniques are applied in the form of Feedback linearization, LQR and PI control and the performance is compared based on defined criteria.