Waking dive drags down ships

Aspidochelone

However, as soon as the heat from these fires penetrates its thick hide, the Aspidochelone dives beneath the waves, dragging the unfortunate sailors into the depths.

Lyngbakr

When enough weight has accumulated on its back, or when the Lyngbakr decides the time is right, it suddenly dives deep into the ocean, throwing off the sailors and dragging their ships down into the depths.