A lot of those busses would prolly start up and run if they're only sitting in 3ft of water. The oil pan is prolly 2-3ft of the ground, so little of the engine would be submerged. There isn't much electronics with diesel engines. The only thing I'd be concerned about would be the starter.
Good observation. I've been thinking the same thing. I doubt that the air intakes are under water. A starter will still crank an engine while underwater too. I have had had to start a few semi-suberged diesels in my time.