Most standard hearing aids are not waterproof and should not be worn out in the rain or in the shower otherwise the rain or water will damage the hearing aids.
If you wear hearing aids and need to go out into the rain it's best to remove the hearing aids and keep them out of the rain until you get somewhere dry and then put the hearing aids back in.
You can buy waterproof hearing aids though so they do make waterproof hearing aids and hearing aid covers to protect them from rain and water.