A supra pubic catheter can cause bladder infections more or less like a normal urethral catheter, and/or skin/soft tissue infections at the port of entry.
It is nearly impossible to waterproof the port of entry, at least in my experience. So-called waterproof dressings don't work well in this situation, and sometimes they increase the risk of infection trapping water and keeping the wound wet for longer. Some doctors would not even recommend normal baths to avoid possible contamination.
A dry suit would obviously solve the problem, but it may be a bit too warm to snorkel in tropical waters. If you don't mind, that's the solution.
The final recommendation should be up to your doctor though, and he may find that a short snorkel followed by fresh water rinse and careful cleaning represents an acceptable risk. If you decide to go that route though, and you snorkel in a foreign country, make sure to ask your doctor to prescribe some antibiotics you can take with you, in case you notice any early symptoms of infection. Last but not least, keep in mind that an infected catheter would need to be removed and replaced, so only travel to countries where you would be comfortable having that procedure done locally, in an emergency.
