Everyone at Elsham on Sunday has fired a paintball underwater.
Now for the science bit.
1) in space, you couldn't fire a standard marker as all moving parts would seize from the cold and your bottle wouldn't cope well without atmospheric pressure. Also your paintballs would freeze solid and nothing would feed. On a brighter note, your paintballs would stay round. Standard batteries/powerpacks would also cease to function. In addition, if you could get one to fire, you'd be sent backwards with each shot giving you a little more acceleration: not good.
2) Underwater: too much resistance, so they wouldn't travel far enough to do a lot. (having to push too much water out of the way). nevermind the whole swelling chopping thing.
I like this thread, it's really stupid.