Adults don't have disposable swim diapers like a baby does. There is ONE brand out there which claims it is, but really is no different than a depends pullup with no sap crystals in it. The reusable swim diapers we do have don't have any absorbent padding in them either, they're only for bowel containment.
And actually, I've found that most pocket diapers have a thin terry lining they use to create the pocket for added liner pads. When worn without the pad, they are able to hold back just enough pee to get you in or out of the water without leaking all over the place. They work really good for this, AND if you plan to spend a little time sunbathing first you can always start withcthe pad then remove before getting in the water (and/or adding it back in afterwards).