In general, the best practice is to store instruments at temperatures above freezing (0 degrees C), and below 35 degrees C (keeping out of direct sunlight while on deck whenever possible).
It is best to minimize exposure at high temperatures, as this could contribute to instrument drift.
At low temperatures, below freezing, the conductivity cell is susceptible to cracking, so it is recommended to store the instrument dry. Note, if the CTD has auxiliary sensors installed, storage recommendations can be found in the appropriate manual for individual sensors. Some instruments must be stored in a liquid solution or in filtered seawater, but this will be made evident in the documentation provided to you upon purchase. It is crucial that these sensors are protected from cold temperatures that may cause these liquids to freeze.