a 200-yard wide asteroid impact will be like a sizable nuclear detonation.
and if it hits the ocean, it would create a massive tsunami that could drown/kill millions across the globe.
No, it would not. I'm getting really tired of the people who have in the last ten years just discovered tsunamis and decide that the end of the world was going to happen from a wave; they're the same people who just discovered that caldera super-volcanos and have decided Yellowstone's getting ready to blow.
To create a large tsunami you have to couple energy from the event into energy in the deep ocean. The dynamics of the deep ocean are at very low resonant frequencies, with a period on the order of many seconds, perhaps as much as a minute. The things that really excite the ocean are events that happen on that frequency. Earthquakes under the ocean often cause one-way shifting of huge blocks of rock some meters up or down; under the ocean that means either a drop or a rise of such a block, permanently displacing an equivalent amount of water. That displacement is permanent, so the ocean has to compensate by moving cubic kilometers of water kilometers within minutes; that causes a tsunami. What doesn't cause a significant tsunami in the open ocean is something fast and transient, like a landslide, or a small asteroid.
An asteroid like that would more likely act by essentially drilling a hole through the ocean. What happened at the bottom would be key; if it triggered an earthquake that moves blocks, that could cause a tsunami; if it simply disintegrated against the rock, probably not much. What, for example, is the volume of the Barringer crater? About .2 cu km; not very impressive compared to even a moderate underwater earthquake. A larger meteor is different, as its energy evacuates a huge volume of water.
Now granted, I'm using an analogy from engineering about energy coupling that was semi-instinctual in my practice of engineering; no one has witnessed any huge landscape or small (OP sized, anyway) meteor landing on a deep ocean, so what I have here is mainly feelings. But try this: drop a rock into a pond (not a puddle) and witness the waves. Then take another rock of the same size and throw it down into the pond with as much velocity as you can. Describe the differences in terms of the waves, say, 5 meters away from impact; did it create waves of a size commensurate with all the extra energy you pumped into it?
Another example: the Crossroads Baker nuclear test occurred inside the lagoon of the Bikini atoll, about 3 km from the shore. The floor of the lagoon was about 54 meters down; the blast was 21 KT. The explosion excavated a 300 meter bubble in the water, most of which came back down from the blast column within a minute. The train of nine waves on the shore 3 km away measured about 2 meters high at maximum. That was a huge fast energy transfer on a shallow pond, and even there the waves created were relatively negligible.
