Monkey Island, properly Cat Dua, is a small karst island in Lan Ha Bay with a clean crescent beach, a basic beach resort, and a permanent population of around 30 rhesus macaques that have grown habituated to tourists. The boat ride from Cat Ba's Beo Wharf takes about 15 minutes.
The monkeys are entertaining but can be aggressive about food; keep snacks and small valuables zipped away and do not feed them by hand. Beyond the macaques the appeal is simple: clear water for swimming and snorkelling, the option to walk up a short cliff path for views over the bay, and a quieter beach experience than Cat Co. Many Lan Ha day-tour boats stop here for an hour, but if you can come independently you will have it almost to yourself in the late afternoon.