You can use a physical zone. It should be under the mission objects > mission section of the bottom half of the mission tree when you're in the world editor creator (F4).
Then modify the gravitymod of the physical zone. 0 is weightless, 2 is twice the normal gravity, .167 is approximately the moon's gravity and -1 is "anti-gravity." Keep in mind that no or extremely low gravity are extremely annoying and will make it nearly impossible for players to do anything. Building in 0 gravity is not nearly as fun as you might think.
You can use applied force to create wind effects or boost effects.
As for using water, if you could provide an example of some settings that provide the effect that would be nice. 0 gravity in water is probably nicer than 0 gravity with a physical zone since pressing shift should still let you go down. I always have trouble getting the blue to go away.
