That’s a lot of nuclear material sitting on the ocean floor.
Yes, but this reactor accident is dumping a lot more into the ocean. It's just diluted by water.
Read the article: it compares the amount of radioactive cesium released by the bomb over Hiroshima to the amount initially released immediately after the earthquake.
On the Ocean floor is not an immediate problem. The cold water down there barely moves and takes decades to reach the surface. Only giant squids or whales make the trip up and down quickly in the biological area. Shallow water release means shallow water organisms we eat get contaminated and the nuclear particles can be ejected into the atmosphere by waves and bubbles. Not the same concerns at all.