Hideo Tsurumaki watched the giant tsunami crash onto Japan's northeast coast on March 11, 2011, sweeping away cars as people tried to escape from them.

As the vehicles sank slowly into the sea Tsurumaki thought about his mother, who lives by the ocean in another part of the earthquake-prone country. Like many of those who tried to flee by car that day, she has difficulty walking. And he thought that if the cars had been able to float, fewer people would have perished.

Two years later, the former employee of a unit of Toyota Motor Corp. started to build a small, watertight electric vehicle that can float in floods — or even cruise, at low speeds. His startup has some notable backers, a functioning prototype, and plans to produce 10,000 cars a year from late 2018 at a factory that it's renting near Bangkok. By 2020, Tsurumaki hopes to take the company public.