Most of the protected islands in Costa Rica have no landing 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.