Despite Nicaragua ’s gorgeous shorelines, diving is a pastime only beginning to be developed, with only three shops in the whole country: one in San Juan del Sur  and one on each of the Corn Islands.
Snorkeling is best off the Corn Islands and some of the Pearl Cays. While the Miskito Cays might have good diving as well, they are unfortunately inaccessible. Start on Corn or Little Corn Island and do your part to encourage preservation of these still beautiful but delicate ecosystems.
For freshwater diving, inquire at Laguna de Apoyo , which is deeper than you’d guess by looking across the glassy surface of the lake.
Corn Island and Little Corn Island share a combined 12 kilometers of reef. Each island has a small, personable dive shop to equip divers and take them to the sites. Both islands’ reef systems feature a stunning diversity of wildlife, including rays, eels, angels, groupers, sharks, and enormous pools of African pompano. Both shops offer PADI certifications for about $250, plus a range of packages for all skill levels.
Although the diving off the Corn Islands is impressive, the most spectacular and undoubtedly world-class site is found nearly 25 difficult kilometers farther out to sea around a sea mount called Blowing Rock.
Nicaragua’s Pacific side offers diving too, particularly in the south near San Juan del Sur , but conditions are predictably unpredictable (visibility can change from 1 to 20 meters from day to day). There are rock reefs here (no coral formations) inhabited by large fish, including colorful wrasses, parrotfish, snappers, and huge surgeonfish. Once abundant, shark populations have thinned severely due to overfishing.
The best Pacific diving in Nicaragua is between December and April during periods of clear, cold water upwelling associated with the strong offshore winds. Visibility is significantly poorer during the rainy season (June–November) due to sediment from the rivers, which enrich coastal water and provoke algal blooms.
For freshwater diving, Nicaragua offers the volcanic crater lakes of Apoyo  (near Masaya ) and Xiloá  (near Managua ). The diving in both lakes is part of ongoing biological research of the endemic cichlid fish species, and experienced divers can rent tanks from the folks at the Proyecto Ecológico in Laguna de Apoyo.
Apoyo has better visibility, but Xiloá has more colorful fish, and is especially interesting during the peak breeding season in November and December. There are still at least 10 undescribed fish species in the freshwater crater lakes, but hurry, as they are being decimated by introduced tilapia.