Puerto Rico is a self governing commonwealth of the United States of America and is located in the Caribbean sea to the east of the Dominican Republic and west of the U.S. Virgin Islands,  Puerto Rico lies on a key shipping lane to the Panama Canal, the Mona Passage.  Here's the map.

Puerto Rico consists of the main island of Puerto Rico and various smaller islands, including Vieques, Culebra, Mona, Desecheo, and Caja de Muertos. Of the latter five, only Culebra and Vieques are inhabited year-round.  Mona is uninhabited most of the year except for employees of the Puerto Rico Department of Natural Resources. There are also many other even smaller islands including Monito and "La Isleta de San Juan" which includes Old San Juan and Puerta de Tierra.

The best diving is off the island's south, east, and western coasts. On the east coast, a chain of small islands (leading toward Culebra) seals the moderate warm waters of the Caribbean Sea from the Atlantic Ocean. 

For more experienced divers, there's the Great Trench just off the South Coast and, between the cities of La Parguera and Guanica, you can dive seawalls and underwater caves.

Mona Island, sometimes called the Galapagos of the Caribbean, is a rarely-visited natural wonder. A  paradise for nature lovers, it is inhabited by rare mammals, giant lizards (Mona iguanas) and endangered birds like the red-footed boobies, and leatherback sea turtles.  Its waters are home to unspoiled coral reefs located at depths of around 80 feet.

• The Mona Channel (off the west coast of the island) is a haven for dolphins and whales between November and May.
• Beneath the lighthouse on Mona Island is another cave network said to rival Camuy, reached via the trails established by guano miners many years ago.
• The island of Culebra boasts over 50 dive sites, including cave dives, a natural tunnel, wall dives, a shipwreck dive (The Wit Power) and endless coral reefs.
• If you're visiting Puerto Rico during the right time of year, you may be able to spot a herd of migrating manatees or a pod of whales.

• Mona Island is 45 miles west of Puerto Rico (or a 6-hour ride in most dive boats) so has to be organised as a 2/3 day camping excursion, for which a permit has to be obtained.  So, if you want to do, plan ahead and don't expect to just turn up and ask someone to take you to Mona.
• No vaccination requirements for any international traveller. 
• No passport required for US citizens.  Citizens from other countries have the same requirements as for entering the USA.