water glass; underwater viewing box
A box-like device with a glass bottom used to observe underwater objects, such as fish or coral, from a boat or shore. Also called a hydroscope.
When you look into the sea with a water glass, you can see the fish clearly.
The children used a water glass to observe the stones on the riverbed.
Compound of 箱 (hako, 'box') and 眼鏡 (megane, 'glasses'), literally 'box glasses'. The exact origin is uncertain, but it likely arose as a descriptive term for the box-shaped viewing tool.