This is the action you need
{
Action = Add
AddTo = "<Ammo/ammo.txt>/Ammo"
ToAdd
{
ID = advanced_vanilla_water
MaxCarried = 1000
CarriesMultiple = 1000
CarriedSprite
{
AnimationFiles
[
"Ammo/water1.png"
"Ammo/water2.png"
"Ammo/water3.png"
"Ammo/water4.png"
]
AnimationInterval = 0.15
Size = [1, 1]
}
CarriedSpriteOffset = [0, .25]
}
}