If you're talking about a person or animal, I would say that the distinguishing point is whether it's entirely submerged, especially the head. Like if you were standing in a pool and the water was up to your neck, then you would be in the water. If you dunked your head under the water, then you'd be under water. Sometimes food is labeled as 'in water.' In that case, you wouldn't refer to it as under water. For general objects, 'under water' typically means entirely submerged. Like a pier is in the water, whereas if the pier was under water, then it would be entirely submerged.

Great question! The phrases "in water" and "under water" are similar but have distinct differences based on context and meaning. Here's a clear breakdown: 1. "In water" - Refers to something being surrounded by or submerged in water, but not necessarily below the surface or entirely covered. - It can imply partial or full immersion. Example: - The toy is floating "in water". (The toy is in the water but not underwater.) 2. "Under water" - Specifically refers to something being below the surface of the water, fully submerged. - It often emphasizes being beneath the surface. Example: - The diver is "under water" (The diver is completely submerged below the water's surface.) Key Difference: - Use "in water" for a broader sense of being surrounded by water. - Use "under water" to emphasize being beneath the water's surface. Tip for Your Niece: Ask her to think of swimming: - If someone is holding their breath and diving, they're "under water." - If they're just wading or splashing, they're "in water."

Here's another one to add to your list: "in deep water". It means "in trouble".

In water suggests either moving or floating in water whereas under water suggests fully submerged underneath the surface of the water

I would also add to Matt T's explanation, that everything is in the water or in the ocean which shows a "general" location. And "underwater" is more specific to things that are submerged completely. Masa, that is an interesting question. I had never thought about that before. Arigatou.