The ocean and its corals are only one type of diving environment. There are many other types of dive sites depending on the area in which you live. Some of the sites may involve extra training, and some may not. Your local dive center is best equipped to advise you regarding the diving in your area.
Many people dive in subtropical and temperate waters in places all around the world. These diving sites have many alternatives to the coral environments that are seen in the tropical areas.
Sponge gardens are very common in cooler waters and have a large variety of colors, shapes, and species. These are often home to many kinds of fish and create an ecosystem that is quite diverse. Sponges are delicate filter feeders. You should take good control of your buoyancy when diving around them so that you do not create any damage.
Rivers and Estuaries
Often, divers enjoy exploring different environments such as freshwater rivers or estuaries, where rivers join the ocean. There will be many different kinds of animals that you do not usually see in the ocean. This can be an exciting type of diving but can involve currents or swift water movement. You may need extra training to dive in some of these areas.
There are many places where the ocean is a long trip away, but a lake with a diving area may be nearby. Some lakes are very clear, but others are not. You may need training in limited visibility diving.
Lakes are usually fresh water, so they will have a variety of freshwater animals to see. Care must be taken near the bottom of lakes because there can be very fine silt that can create low visibility very fast. Keep off the bottom, and keep your fins out of the silt.
Freshwater Cave Systems
Cave systems definitely require extra training but can be beautiful and exciting places to dive. Often, there are colorful formations from a time when the cave was a dry system. Stalagmites and stalactites can be seen as well as a whole new range of animals that live in low-light conditions.