The problem with detecting water on any giant planet is its density. It's a lot denser than the normal atmospheric constituents, even when they're in their liquid forms or solid forms. So, it gradually sinks down into their atmospheres to the bodies of these worlds. You will have some in the lower atmospheres though. But it's usually too deep to detect spectroscopically. That's why, when Galileo sent its atmospheric probe into Jupiter, they found a relatively dry atmosphere (at least in that area where it went in). In any case, the probe never got low enough down in the atmosphere to get to the water layers. It was crushed beforehand by the pressure.
But they can tell it's present through lightning. Water is a polar molecule, so it can carry a charge. You need water molecules/ice to create the charge separation in clouds to generate lightning. When convection brings it up through the atmosphere, it forms ice and the charge separation occurs in the clouds.....zap!!!!.....one massive bolt comes flying out
Lightning on Jupiter is rather spectacular....the megabolts that arc between thunderheads on Jupiter are several thousand times as powerful as any on Earth.