1. The switches on an electric shower are designed to be used in a wet environment and remain safe.
2. You could have a ceiling mounted pull cord switch installed to control your shower.
3. You cannot have a normal electrical switch installed within certain areas of a bathroom, and certainly not where it could be reached from someone in the shower.
4. Whether any electronics within the shower would be damaged by power being switched on / off by the pull cord switch I don't know.