The Japanese loved floatplanes for a few reasons. Their war was a chain of islands for defence of the home islands, and advance scouting. It's far cheaper and faster to create small ramps on the beach of an island, than an airfield. Planes could be pulled out of the water between missions. Their float planes were mainly for scouting, and nuisance value. Flying boats of both sides were often based where no airfield could be built.