For reference the Tunguska event, which was projected to be a 5MT explosion on the low end, flattened about 2000 square kilometers when it airburst. If this hits the ocean it would no doubt cause some large tsunamis.
This is why nearly all nuclear explosions was over land or far from coast. Few experiments near some coast or island, created tsunami-like wave on those coast (island), which considerable widen polluted territory (ocean is usually considered as self-cleaning).