In a worst case where all the fuel simultaneously becomes a suspended powder, would kill a bunch of sealife in an area smaller than 100m radius. We do far worse on a regular basis with commercial fishing.

You make a sea reactor dump sound like China Syndrome. The fact that 11+ of them happened with no significant problems makes it pretty clear that you're exaggerating.

Microplastics and plastics in the ocean are a far more serious and far more tangible concern than a hypothetically hijacked ship hypothetically dumping the core which hypothetically turns into powder form.