Am on bed rest and the doctor advised me to go and exercise in the pool to get prepared for labour. Besides walking in water, what other simple exercises can I do?

On top of what that has been said, you can consider attaching a thick rubber band to provide resistance for shoulder and arm exercises. Lower body exercises are also important so simple movements like circling the ankle and bending the knees and pulling the ankles towards the body will be good forms of simple exercises.