Freshwater pearls are not completely impervious to water. The nacre of the pearl is an organic substance, so while it might appear strong and durable, it will deteriorate over time due to exposure to chemicals in tap water. This can cause the nacre to degrade and peel away from the surface of your pearls.
No, you can not wear your freshwater pearl jewelry in the shower. They are too delicate to withstand the harsh chemicals and temperatures of a shower, therefore they will deteriorate faster than other pearl types. Pearls are also very sensitive to pressure, so it's important not to sleep in your pearl jewelry as well.
Also, you want to avoid exposing freshwater pearls to lotions and perfumes. These substances contain chemicals that will change the surface of pearl jewelry rendering it matte or tacky. Also, avoid wearing them in chlorinated water since they are not chlorine-resistant.
If your pearls do get wet, make sure you pat them dry with a clean, soft fabric and allow them to air dry before storing them away.
To preserve their beauty for as long as possible, store your pearls away from other gemstones that might scratch them. Keep your pearl jewelry wrapped in silk or plastic and place them in a compartmentalized jewelry box.
You should wear freshwater pearls only when you plan to remove them at the end of the day, this should minimize their exposure to chemicals and prolong their life.