I'm not saying this is ideal. Ships shouldn't be simply scuttled at the end of their life. They should go to scrap yards. However, an isolated incident far from any fishing areas far out in the sea isn't going to be environmental disaster. There are many large ships at the bottom of the oceans. E.g. just count sunk WW1 and WW2 battleships.

At least there was attempt to do it properly with this one ship, but it didn't work out.