you just need to carry alot of those waterskin/canteen with you. Obviously outside The Water Bender's Element they have a slight disadvantage. If the watertribe ever intends to win a war They have to bring with them a Large supply of Water on dinos to win.

You only need 1 empty waterskin

I was not aware that you could bend without a filled container. I've only tried with filled water jars because I added those to my beacons. But I have noticed the water bending does not actually consume water charges from the container, so you don't need to bring a ton unless you plan on dying and needing to re-up. However, if you look at things logically, water benders in a world like this would be at a disadvantage. Especially if you're on Scorched Earth or The Island. Regardless of how much water you carry you couldn't possibly carry enough to use any of the current water bending abilities except the projectiles. The others would require vast amounts of water to use.

As far as balance is concerned, GyozaGuy has added configurations for each skill for each element. Rebalancing the bending for your server should just be a matter of testing different configurations. On my server I started by reducing the chi cost of everything to 25% and then worked up from there. http://psteamcommunity.yuanyoumao.com/workshop/filedetails/discussion/729171778/1318835718936737505/