I guess this is a two part answer prefaced with "It depends upon what your looking for". If your looking for the little style disposable with prints which I gather you are from your previous experience with splashers, simple answer to that is... You won't find them. A good substitute is a pull up style cloth backed brief like swim mates from northshore. If your looking for function only (I.e fecal containment ((which really is the only purpose of a swim diapers)) then your probably looking at a plastic pant option. I don't remember the exact the company but they marketed a re-usable diaper specifically for swimming. (And they were ugly looking) If your somewhat of the handy type, I bet some silk screening or model dye might get the look your trying to achieve. There is also a guy on eBay that makes custom diapers from commercially available baby ones. Alas, I never ordered from him so I don't recall his name but maybe another member does and you might try reaching out to him.
Good luck in your adventure.