The sun's spectrum doesn't have the most energy in the visible light band, though it's close. Most of the energy is in the infrared band:

https://sunwindsolar.com/blog/solar-radiation-spectrum/?v=0b...

Both the "because that's what the sun emits" and "because we are mostly water" explanations are incomplete. There are plenty of other animals [1] that can "see" infrared.

The real reason is simply because that's how we evolved. That's how the "because those are the frequencies that pass through water" explanation comes into play: vision first evolved in aquatic animals, so frequencies that don't penetrate water wouldn't have been all that helpful to their survival and reproductive success, and so wouldn't be selected for. But that's incomplete too: salmon are one of the top IR-sensing animals and they live in water, so when there's an evolutionary need to select for IR vision, it happens. The reason we "see" in the visible light range is simply that that's how we've defined "visible".

There are some physics reasons as well, notably that most mammalian body structures emit heat, which would blind an animal that relies on infrared to see (notice how most of the animals that can see infrared are cold-blooded reptiles, fish, and insects), and that most of the high-resolution biochemical mechanisms that can convert electromagnetic waves to electrochemical nerve impulses operate in the visible light range. Structures that convert infrared radiation to nerve impulses are more complex and more costly to support, so unless there's a clear survival benefit for the species, they tend to get selected away.

[1] https://a-z-animals.com/animals/lists/animals-that-can-see-i...