Yes. Getting rid of the fluid will the heart to pump better and more effectively. This usually gives dogs a lot of relief in the first few days.
It does sound like you are describing heart failure, particularly since you are seeing a decline in energy, movements and a cough.
The next step, if you want to find out how much damage there is to the heart is to get a cardiac ultrasound. This is the best diagnostic tool and can help you figure out where to go next.
Let me know if you still have questions.