Suggest some uses for a plastic that disolves in water
Packaging something that can't/shouldn't be handled but you want to put into water. A suggestion could be radioactive or biologically contaminated clothing that needs to be transported/handled on its way to being washed. To stop the operator becoming contaminated the clothing could be in a water soluble plastic bag and the operator puts the whole thing into the washing machine. The bag dissolves and will go with the waste water, but the clothing can be washed.
Here is an idea that is not in use as far as I know. Many clothing shops have a problem of returned goods that have been worn & washed. If there was a label with an opaque plastic cover which came off when the items was washed this would alert the shop assistant.