Dear Tír Na nÓg ,
thanks for this brilliant Question, I really like it.
It simply through a process called osmosis. The denser fluids try to disperse into the less dense. So a freshwater fish having its internal organs being more dense than the water around it would have to expel excess water to prevent over saturation of the cells. While in salt water fish the case is different. The surrounding water is denser thus the fish must literally drink like a fish to prevent dehydration.
Yes. Fish that live in salt water lose moisture from their bodies through osmosis. These fish need to drink lots of water, or they’ll become dehydrated. The extra salt is then simply excreted.
hope it helps