To get a water bottle in Minecraft PE so you can add it to your inventory in Survival mode requires 1 glass bottle and a water source. Select the glass bottle so you are holding it in your hand then stand in front of your water source and pick up the water with the glass bottle by tapping the water. When you do this the message 'Water Bottle' will appear on the screen and your glass bottle will turn blue with water. The water bottle ID is 373 and the /give command for Water Bottle is /give @p potion 1 0. Water bottles can be used to brew Items with a brewing stand as well as transport water.
The video below is a tutorial on how to get a water bottle