The trip to Bonny is part of the visit. Wikivoyage describes the approach from Port Harcourt by speedboat as about an hour, with ferries and boats serving as the island's main transport link.
This is not a sightseeing cruise in the polished sense. It is a practical run through the Niger Delta's waterways, past fishing traffic, mangrove edges, and the industrial infrastructure that made Bonny a name in oil and gas. Use reputable local operators and take current safety advice seriously.