Why does the water in the pool look a different color than in the glass?

Ah, good question, that's something that happens to me constantly from a trainer's perspective. Basically it has to do with depth and how light interacts with water. When you fill a glass, there's so little water that light just passes through it without any problem and you see the bottom of the glass clearly. But in a pool, especially if it has some depth to it, light has to travel a longer distance through the water before it comes back to your eyes. In that process, the water absorbs more of the red and orange colors (the more "warm" colors), while it reflects and scatters more blue. That's why you see that intense blue color in the pool.

The total volume of water also plays a part in it. With a lot of water, the effect gets amplified a ton. It's like the "blue" accumulates with depth. Plus, depending on where the sun is and how it reflects, it can look more blue or more greenish. Some days when I train early in the morning, the water looks almost gray, but at midday when the sun hits directly, it's a really intense blue.

If you wanted to experiment more with this, try a bucket or a deeper container with pool water and you're gonna start noticing that the more depth you have, the more blue it's gonna look. It's the same thing that happens in the ocean, right? The deeper it is, the darker blue it looks. That's why on beaches with crystal clear and deep water you see that beautiful turquoise blue, but if it's really shallow water, you just see it transparent.