I was primarily a swimmer for decades and took up cycling when I realized that my legs needed more strengthening.
Even with a fin and kickboard, I didn't get my legs conditioned other than from pushing off the wall at turns.
My core muscles were very strong and my cardiorespiratory system was in great shape.
Breath control I got from swimming has served me well on the bicycle.
The upper body exercise from swimming has helped me on the bike - anchoring my body climbing and holding an aerodynamic position in the drops.

But just about any kind of exercise is better than the monotony of a trainer or a treadmill or just sitting on the couch.

I suggest alternating exercise so in addition to swimming, include some time on a stationary bike and doing some weights.

Lower back pain in swimming can come from hyperextending during butterfly swimming, some drills, kicking with a kickboard, and flipturns.
Poor technique in the freestyle and breaststroke also can stress your lower back.
Swimming backstroke will correct muscle imbalances and is easy on the back.

As far as weight loss, you can burn plenty of calories while swimming and lose weight. Just like cycling. I always have to watch calorie input in both activities to keep from gaining.