Boil is doing a lot of heavy lifting there. It wouldn't disappear -- it has no where to go. It would just get very cold, which is essentially what you would expect of anything exposed to the vacuum of space. It wouldn't turn to ice, though, it would turn to vapor -- thus the "boiling". It would still, for any functional purpose, be a large amount of water lingering around the immediate vicinity of his head.

I assume it would depend on the amount of water. I feel like if it is enough water to fill a helmet to the point of threatening to drown, the expanding water itself would probably block up any small vent or opening in a helmet before a meaningful amount of it was evacuated. I'm not a space man though so it very well may work.

The reason explosive decompression can be a thing in a plane but not in space is because of the force of outside air acting on things inside of the vehicle. If something acts on an air plane to abruptly depressurize it through a sufficiently large enough hole, it isn't so much that things are "sucked" out of it as it is that things are "blown" out of it. It is the same force that makes it feel like something can get sucked out of a car window when your vehicle is in motion even though there isn't actually a meaningful pressure difference between your closed car and the outside air, it has to do more with the motion of the air along the vehicle.

In a vacuum things like gasses (air) would expand outwards through a breach, but there isn't the same force acting on a space shuttle that there is on a vehicle in motion on Earth, the popular media idea of things getting explosively decompressed out of a spaceship wouldn't actually happen outside of the force of whatever caused the breach to occur in the first place.

The force exerted by air expanding into a lower pressure area, like what would happen if you "opened the car door" so to speak, but in outer space, isn't enough to actually "suck" things out of the vehicle. Except probably really light things like paper or something, depending on how abrupt the breach was and how big (or, small) it is. You'd basically have to get sucked through a garden hose to generate enough pressure to drag a human body out of a spaceship -- and in that specific theoretical you'd block the opening with your clothing or just the weight of your body far before any actual bodily damage would occur. It probably wouldn't be fun to experience but you'd survive and with all your limbs.

Yeah all the comments acting like instant death would occur if any part of your body is exposed to vacuum are exaggerating a lot. It would certainly be unpleasant, possibly damaging to tissue, but the mortal danger of exposure to vacuum is way overexaggerated by popular media.

The actual danger of removing his helmet in the situation above is that it wouldn't solve the problem, because the water would just linger there with no outside force directly clearing it off from him. It'd be pushed in all directions due to venting gas from his suit presumably, but the water would be expanding at that point anyways. If he just slaps his helmet back on it would, presumably, still contain a large amount of water and continue filling with water.

I live in Michigan now but I was born and raised in Florida. A lot of my coworkers or friends that are native to Michigan will ask me about hurricanes when I mention I grew up in Florida, and I'll comply with a horror story or two of losing power for weeks at a time or having water flood up to our front door or trees falling around the house, all the usual things. People are usually amazed anyone can live through stuff like that.

The actual secret is I grew up in Tallahassee. It was that bad there, and we might as well be in middle Georgia for how much hurricanes actually affect the area. Living by the coast you might as well kiss your property goodbye. There's not much long term you can usefully do.