Most of the protected islands in Costa Rica have landing prohibition policies in effect. Visitors are welcome to cruise offshore photographing birds and other wildlife and boat dives are allowed at most, but the islands themselves are off limits.
These strict regulations are enforced because the islands protect such delicately balanced isolated ecosystems. Impacts on endemic (exist nowhere else) species by intentionally or inadvertently introduced species can be disastrous as evidenced by the rats and goats on Isla del Cocos.
A couple of islands included in the protection of larger National Parks (rather than having a designation of their own) are worth noting. Isla Pajaros (Island of the Birds) in the Tempisque River is part of Palo Verde National Park and offers excellent birdwatching. Islas Murciélagos (Bat Islands) offshore in Santa Rosa National Park is one of the premiere dive locations in Costa Rica.
Isla de Caño was a separate biological reserve until it was absorbed into Corcovado National Park and is the most popular island to visit. Snorkeling and whale and dolphin watching boats from all along the southern Pacific coast anchor in the bay. Overnight stays are not allowed but you can land at the amazing beach near the ranger station on day trips and there are two short trails through the rainforest to pre-columbian stone spheres and an overlook on the opposite side of the island.
If you are planning to visit please check with your tour operator for the current status because from 2012 to present there have been a number of issues that have resulted in temporary closures of the trails, bathrooms and even the prohibition of visiting the beach.