Pool time is a great way to both relax and raise your activity level.
According to Shape magazine, pool exercises are good for everyone — and great for those with back or knee pain.

For cardio and total body exercise, nothing beats laps across the pool. You can use a variety of styles and work on both speed and distance.
Other options to get your heart pumping include water jogging, bicycling, sculling, and jumping jacks. For bicycling, prop yourself up on the side of the pool and move your legs like you are pedaling a bicycle. Sculling involves curling your legs up into a ball and keeping yourself afloat on your back as long as possible by paddling with your arms.

Pool weights for resistance
To get the most out of upper body resistance movements, you’ll want to have access to pool weights or at least some pool noodles which can be purchased in a sporting goods store or found at some public pools. As long as you keep the weights underwater, you can do most traditional gym moves: bicep curls, lateral raises, tricep extensions, and even pushing and pulling to work out your back and chest.
With a pool noodle, do press-down work for your chest and triceps as well as underwater planks.

Lower body workouts
Lower body resistance work such as jumping or running is easier on the back and joints when you are in the water. Experiment with jumping out of the water as high as possible in a variety of stances to get a feel for what works. The wider your legs, the more you will engage your glutes and hamstrings, while narrower positions will work your quads. For inner and outer leg work, stand near the wall with your hand out to keep steady and then extend one leg at a time out to the side and back again as quickly as possible to maximize resistance.