There was a long list of engineering failures at Fukushima. The idea with airplanes is not "design this component so it cannot fail" but "design the system so it can tolerate component failure". Fukushima had a list of failures it could not tolerate.
You mentioned one, the vulnerability of the backup power to the seawall being overtopped. The generators could have been put on a raised platform. There were others:
1. the hydrogen was vented into an enclosed space
2. no way to add water to the cooling system with a gravity fed device
3. critical machinery should not be located in the reactor core building
4. no way to bring in electric power from elsewhere
You mentioned one, the vulnerability of the backup power to the seawall being overtopped. The generators could have been put on a raised platform. There were others:
1. the hydrogen was vented into an enclosed space
2. no way to add water to the cooling system with a gravity fed device
3. critical machinery should not be located in the reactor core building
4. no way to bring in electric power from elsewhere