Although technically the whole ocean is water, it is filled with contaminants that render it useless for research and crafting.
Purified water can be obtained from chests in aquatic environments, which will routinely purify samples from their surroundings. To collect the pure water from the chest, simply revisit it when it is glowing.
Each chest can only purify so much water, and the amount increases the deeper the chest is located.
Can be found at:Edit
Aquatic Planet: 0-250 depth
It can be harvested near the capsule. The chest will produce the same amount of pure water every 5 seconds and if you need a lot you can just stay on the chest, no need to move.
Water can be electrolysed from H2O into 2 H2 and O2. These form an ideal mix for combustion, when they reform into water. Therefore, water cannot be used as a chemical fuel directly. The components can be used to fuel nuclear fusion, although this is presently impractical as a power source.