This has been my first summer managing incontinence, and we typically do a lot on or near the water. I have used both Swimmates and an SOS reusable swim diaper (not at the same time). As mentioned above, swim diapers are designed for bowel incontinence, but have enough absorbancy to conceal urine leakage for a short time before you get wet. I have had to change my behaviors a bit if I am not going right into the water. I will wait to change into a swimsuit until just before swimming, once wet it does not really matter (just try to keep your suit wet). There have been days where I just hang out by the water in shorts so I can stay fully protected and no one has asked why I did not change.
I will say both products fit nicely under my swimsuits, but the waistband on the Swimmates can become visible if the drawstring is not tight. I had my first accident in a Swimmate friday night. Though I had been swimming in the lake, the accident occurred on shore. Containment was good, but I am not sure what would have happened if I had sat down or been in the water.