No restrooms on the islands — use the visitor center.

That is almost always the case. Most island parks (like Dry Tortugas, Channel Islands, or the Apostle Islands) have zero plumbing past the main visitor center. A few might have a vault toilet or a composting toilet, but you cannot count on it.

It comes down to logistics: pumping sewage on a remote island is expensive and environmentally tricky. So parks concentrate facilities at the mainland or the main dock. The islands themselves are left wild.

Plan accordingly. Pee before you board the ferry, pack hand sanitizer, and don’t assume there will be a porta-potty unless the park website specifically says so.