Swimming is DEFINITELY good cardio. It burns a lot of calories, uses lots of muscles, and it is no-impact which makes it very safe (assuming you keep the water out of your lungs). However, there is arguably one major drawback to swimming for fat loss. There is a phenomenon in exercise science regarding swimmers and body composition.
Here goes: for the amount of calories expended, swimmers tend to not be as lean as other athletes who expend the same number of calories on land (running jumping etc.). 
One theory is that because swimming is done in the water, the body prefers to keep a certain amount of subcutaneous body fat in order to help keep you afloat.
After all, having lean muscle and no/low body fat is NOT very conducive to swimming; it’s more conducive to SINKING!
As anecdotal evidence of this, think of runners (especially sprinters) versus swimmers. Practically all sprinters are shredded; yet no swimmers are shredded. Sure, they are often really lean, but NOT shredded.
Although I certainly do NOT want to discourage anyone from swimming for exercise, if you’re a pre-contest (bodybuilding, fitness, figure, bikini) competitor, I wouldn’t do it more than one per week or so.
Yours in health,
Dr Clay
Have you seen a picture of me recently? I’m not shredded???
Yeah, Michael, you’re pretty dang lean, that’s for sure. But, I wouldn’t say “shredded.” That’s a term I reserved for someone (like a good competitive bodybuilder on contest day) who has striations in the chest, delts and maybe quads and glutes. You have separation between muscle, but no striations (that I see).
And for the record, you are far leaner than most swimmers; so I’d say you’re an exception to the rule.
Swim on, Michael.
Dr Clay