If I were to guess what the most abundant molecule was in the whole universe, I would have to say that it is water. Of course there is probably more free hydrogen and oxygen than there is the combination of the two into the water molecules, but they are simple molecules and water is a little more complicated. Yes, water is quite abundant out there in space and on other planets and moons. One of Jupiter's moons spews water from, yes, water volcanoes. Here is an artists conception of what they would look like:
This painting shows the icy moon's volcano with Jupiter in the back ground. The Hubble Space Telescope took pictures of icy volcanoes on Europa (a moon of Jupiter) and also on Enceladus (a moon or Saturn). Our solar system is a wondrous place and the universe is ever more wondrous. Our Earth is just a microscopic speck in our universe and theory has it that there are many more universes. Live every day in awe of the beauty and wonder that is all around us and have a great day, you hear?