When a barrier is placed in indoor terrain, it's floor to ceiling and blocks everyone. When a barrier is placed outdoor, it's ground to "kinda high" and fliers can freely pass over it while others must go around.

If you imagine these barriers as roughly an 8ft square wall, it answers the water question to me. If it was to be placed on water terrain in an "at sea-level" style placement, all characters could just swim under it at their normal swim speed. If it was placed under beneath the water line deep enough to render that at least "more difficult", it would have dropped the height enough to no longer block line of fire, as well as start giving people the option on the water's surface of just climbing over it.

If this is a debate for the purpose of more realism or comic accuracy, I understand that characters should be able to place the barrier there, but it would not operate under the same game mechanics (which is a large part of what Special Powers is for).

I suppose a change could be made causing barriers placed over water terrain to act as 2 units of the barrier (one barrier section placed above water and one below), but I really don't think the benefits of the change would outweigh the additional complication of rules. For ease of gameplay, I'd leave it as is.

Cyclops: Iceman, put a wall of ice to block those Atlanteans.
Iceman: Scott, they're Atlanteans. They'll just swim under it. Why don't I throw some spikey icicles at them instead.
Cyclops: Sorry, bud. Stupid idea. I shouldn't bark orders when I'm off my leadership click.

you can put smoke on the water
*ques song: duh-duh-duh. duh-duh-di-duh. duh-duh-duh-diduh*

Everybody seems to focus on the characters who create their
barriers out of ice or fire or rock, but a huge number just create
"force fields" like the GL's and Invisible Woman. These characters
probably make up the bulk of barrier users, and they should not
be affected by water at all. That's why I voted yes.

I'd say to balance things out, allow people with the
water movement symbol to be able to pass through them
(as swimming under) when in water terrain, just like a flyer
can fly over them.

Or better yet, just keep these points in mind for future sets, and think about building fewer restrictions into certain character's versions of Barrier. Really, this is probably the best way to handle it. With that said, I still voted yes.

Darn you, I like the song, but now i can't get the song out of my head... AAAHHHHHH!!!!

I don't see why characters can't place barrier tokens in water terrain. I hope this rule is overturned even though I don't think the situation has ever come up in a game I've played. I normally very rarely use barrier and most of my players and friends don't really use it much either. But, it should be allowed in my opinion.

Seeing as it is an action to use barrier, doesn't that show that the character is exerting and can effort to keep it up?

On an unrelated point, I don't understand why objects can only be placed in clear terrain.

I can see how someone could put a barrier up above the water, but you could always swim under it. Who's powerful enough to put up a barrier all the way down to the bottom of the ocean or lake? Not many characters. And the one's that could do it, why would they? It seems like an exhausting use of powers.

Maybe this is a good time for me to raise public outcry about this... Why can a character that's been TKed make an action that turn, but a character that's been carried can't? I mean really, what's the difference? And if it were me, for example, I would be more likely to do something impressive after being plopped down on the battlefield by the Hulk than after being moved through the air by someone's mind, the only action my fragile mind would be able to signal my body to do would be to wet myself. So who's with me? Let's do a poll on Carry and Action.

The short answer: game balance.