It depends, on where exactly will hit. Similar to nuclear explosions, if one happen over sea or ocean near coast, it will cause extremely large wave, which will be similar to tsunami. If it will hit land, will be small earthquake.

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).