Heya,
Watercooling still requires fans, and the radiators are huge and often have two or three fans on it. Watercooling isn't quieter. It's just something else to do, fun, etc. Something to spend money on. You'll find there's not a big difference in temp control for the common user, it's only at the extremes that you find differences. And of course, the idea of the watercooling setup having the CPU, GPU, etc, all in a circuit doing this.
Passive cooling can be done; but it's not easy. The CPU/GPU get hot under 100% load. A fan is not needed, but thermal handling is needed, so to do that you'd have to have a way to carry the heat away from the CPU/GPU. The heat sink would have to be enormous. And even then, you'd be targeting the high side of tolerance for the equipment. You can find heat sinks for this. They're monstrously huge, which requires a huge case. But then the problem is also the case, because you need air flow through the case, which won't happen without a mechanism to produce positive/negative pressure so that air flows through the gradient. This won't happen without a fan somewhere.
Thing is, a big CPU cooler can be dead silent with a fan on it. Just use 120mm fans. Slow RPM. They're quieter than the standing noise in your house.
GPU's get loud, no matter what. They get hot.
But you can game with an APU, like a latest release Ryzen APU that will have all the CPU/GPU in one chip under one huge heat sink with two big slow quiet fans. It would be just as quiet as a completely passive system, but way easier to build without specialty gear (look to laptops for guidance on specialty gear to build with really).
And of course, these days, a phone/table is used for gaming a lot, and is completely silent for operation with no fans. Depends on the games you're talking about.
So really, unless you're into top end graphics and competition, just look to a Ryzen APU, get a huge heat sink with a big quiet fan (120mm). It doesn't have to be pricy. The Hyper EVO 212 with a Noctua quiet fan is dead silent. I have three running in my office right now, can't hear them, lots of fans running, but can't hear them, running 50% RPM on most of them. The only device I hear when things are kicking is my GPU (1660 Super with two big fans, but they're loud and I cannot replace them).
Very best,
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