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The fusion of several images of the same scene into a single and larger composite is known as photomosaicing. Unfortunately, the seams along image boundaries are often noticeable, due to photometrical and geometrical registration inaccuracies. Image blending is the merging step in which those artifacts are minimized. Processing bottlenecks and the lack of medium-specific processing tools have restricted underwater photomosaics to small areas despite the hundreds of thousands of square meters that modern surveys can cover. Large underwater photomosaics are increasingly in demand for the characterization of the seafloor for scientific purposes. Producing these mosaics is difficult due to the challenging nature of the underwater environment and the image acquisition conditions, including extreme depth, scattering and light attenuation phenomena, and difficulties in vehicle navigation and positioning. This paper proposes strategies and solutions to tackle the problems of very large underwater optical surveys (gigamosaics), presenting contributions in the image preprocessing, enhancing, and blending steps, resulting in an improved visual quality in the final photomosaic. A comprehensive review of the existing methods is also presented and discussed. Our approach is validated by a large optical survey of a deep-sea hydrothermal field, leading to a high-quality composite in excess of 5 Gpixel.