Actually swimming and aerobic Water exercise are the best for hurt knees.
Other cardio exercises like walking, running, jogging all exert great impact on your knees, which is going to cause more injuries to you.Since you say can't access to a pool, the nearest alternative is for you to do gentle pedaling on an exercise bike or slow biking in the open air.
This exercise cushions a great deal of the impact on your butts, hence less on your knees.
Hope this will help you!!