Renewable energy has been paid attention to in Vietnam in particular and and worldwide in general. Besides wind energy, solar energy is one of the top-priority energy sources. For offshore fishing vessels, freshwater source serving daily fishermen's activities is one of the most crucial and necessary factors. This article presents the calculated results of equipment design and manufacture, which converts seawater into freshwater with a capacity of 5 liters per hour to meet the domestic water demand of fishing vessels and about 20 fishermen using renewable energy (solar energy). This equipment helps offshore fishing vessels and fishermen be proactive in water sources, saving the capacity and load of the ship as well as the cost of fishing trips. The research result is a system turning seawater into freshwater used for small and medium-sized fishing vessels. This is the basis for calculating the water filtration system in general, seawater desalination system and renewable energy system in particular, applied to seafood fishing, offshore fishing and actual production.