Colombia is the only country in South America with coastlines on both the North Pacific Ocean and Caribbean Sea.  Here's the map. 

The islands of Isla Gorgona, Malpelo Islands, San Andrés and Providencia are among the most popular locations.

Isla Gorgona  - This former prison island in the Pacific Ocean is now a nature reserve open and offers excellent diving conditions.  The island is covered with tropical forest.  Famous for its poisonous snakes, other inhabitants are monkeys, endemic lizards, pelicans, blue footed boobies, frigate birds and more; diving is the main reason people visit this island.  The prices are low and the equipment is what you expect for that price. However, dive sites are amazing and it is likely you see sea turtles, moreens and whitefin sharks. If you are really lucky you might see a whale or a whaleshark. 

Malpelo Island is an uninhabited island located 314 miles off Colombia's Pacific coast  It is uninhabited except for a small military post manned by the Colombian Army, which was established in 1986.   On July 12, 2006, Malpelo was declared by UNESCO as a natural World Heritage Site.

San Andres and Providencia consists of an archipelago of islands about 775 km (480 miles) north of Colombia and 220 km (140 miles) from the coast of Nicaragua. These islands are part of the UNESCO Biosphere Reserve, and also the TOC (The Ocean Conservancy Organization) which includes San Andres as one of the five key zones for the health of the planet.   A popular dive is the wreck of the Blue Diamond, a medium-sized freighter, which (according to local custom) was impounded in the 1990s by the Colombian government for running drugs before being scuttled as a dive attraction.  Whatever its history, the Blue Diamond is a fantastically enjoyable wreck, full of little nooks and crannies.

• Isla Gorgona - from mid-June till November is whale season.  Male humpback whales can be seen breaching while the females and their offspring come close to the island at the surface to breath.
• Malpelo Island - home to a unique shark population; swarms of 500 hammerhead sharks and hundreds of silky sharks are frequently seen by diving expeditions.
• The Blue Diamond wreck at San Andres - resting not far off the eastern shore of the island in about 35 feet of very warm water (you can get away without wearing a wetsuit), the dive is easy and fun.  The boat cracked and split as it sunk, and there are several swimthroughs.  Highlights include the massive propeller, half-buried in the sandy bottom, and the bow, which has an open hatch through which you can swim, coming out a hole in the side 

Isla Gorgona:

• There is nothing to buy on Isla Gorgona, so take everything you need with you.
• Vaccination against yellow fever, tetanus and hepatitus B is recommended.

Malpelo Island:

• The currents are often strong and diving is accessible by liveaboard only.
• Visitors need a written permit from the Colombian Ministry of Ecology.