The plant was not designed to operate when inundated by water, it was not designed to withstand a tsunami of this magnitude, it was assumed a tsunami would never breach the protective wall and reach the plant, therefore, simple things like protecting the structures from the forces of the tsunami, and waterproofing all of the ducts, vents, doors, etc with controls over when and how long they can be open, were never in place.
Even with diesel failures at a unit, it could still have been safely shut down had the tsunami not hit. One option could have been borrowing power from another unit, but that would not necessarily be required. Diesels are very reliable machines that are tested on a regular basis. Adding a third does not improve the situation as much as you may think, because if two fail at the same time, its more likely a common cause than a different one, and the third diesel would stand a good chance of suffering that common cause as well. That is where testing, maintenance and reliability programs become very important. Also, having diverse means to achieve a safe state is also key.
The key is not placing a plant that cannot withstand a tsunami where it can be hit by one, because designing to withstand a tsunami suddenly inundating the site it really not practical.