A high power density 10 kW/500 kHz three-phase PWM rectifier (Vienna Rectifier) is under development. Due to preliminary measurements and numerical simulations the total efficiency is assumed to be 95% at full load, resulting in power losses of up to 150 W in each multi-chip power module realizing a bridge leg of the rectifier. In order to keep the power density of the system high direct water cooling is employed where water is in direct contact with the module base plate. Based on the measured characteristic of the water pump (pressure drop dependent on water flow) the geometry of different water channel structures below the module base plate is systematically optimized based on analytical expressions which are formulated based on the well-established theory of fluid dynamics. The design optimization is constrained by the desire to keep the geometry of the water channels in a range that allows simple and low-cost manufacturing. The aim is to find a channel structure resulting in a minimum thermal resistance of the power module for a given pump characteristic. In this paper a very simple slot channel is investigated. The dependency of the thermal resistance on the cooling system is calculated in dependency on the height of the slot channel, and an optimized channel height is found under the side condition of simple manufacturability. Discussing the shortcomings of the simple slot structure, a novel metallic inlay structure is introduced and optimized resulting in a reduction of the thermal resistance of the direct water cooling scheme as compared to the slot channel system. All theoretical considerations are verified via experimental measurements. The general optimization scheme introduced in this paper can easily be adapted to other cooling problems.