It takes a surprising amount of energy to form a gas bubble in water. As an example, consider a glass full with some soft drink at rest. Usually there will be bubbles of $CO_2$ going up continuously, but they will all originate from a certain number of points on the glass surface, not from within the liquid. These points are impurities in the glass' surface that ease the bubble forming process in a catalytic way.
As such, when you depressurize the ship, bubbles will form immediately on all surfaces. The bubbles on the surfaces will repressurize the water with their vapor pressure, preempting it from exploding in a vapor explosion. This repressurization will also, slowly, push the water out of the ship, where it will form big giant drops that are again stabilized by the vapor pressure on their surface as the water continues to evaporate. It's more of a tooth-paste-squirt effect than an explosion.
The really nasty part is, that the bubbles will also form on the skin of your species. This means, that any individual at rest will quickly not be swimming in water but be immersed within a bubble of low pressure vapor, powerless to move. And if the individual tries to swim before the bubble forms, it will create huge bubbles itself on the pulling sides of its fins.
This effect is even stronger if you consider that your water dwellers will be warmer than the water they swim in. This additional warmth means that the water bubbles that form on their skin have a higher gas pressure than the water bubbles that form on a cold wall. So the astronauts will be surrounded by bubbles before the ship's cold structure is covered in gas.
However, this temperature effect can also work for good: Assume that you have some machines in the spaceship that give off heat. If those machines are significantly warmer than the people on the ship, the boiling at the warm machines will keep pressure high enough for the people to survive until the machines are surrounded by gas or have cooled to the range of the body temperature of your species.
Ice won't form until enough water has evaporated from a surface to cool the remaining water down to 0°C. And when that happens, all your crew members will be drifting within their respective gas bubbles, extremely likely already dead.