The modern attack submarine is capable of unleashing an accurate barrage of conventional or nuclear missiles against land-based and ocean-going surface targets while also having the facilities to counter enemy submarines of similar design. Additionally, submarines can be used to assist special forces in clandestine operations. Many modern world navies utilize the submarine primarily as a deterrent element in territorial waters with most forces keeping a standard fleet of about five or more boats.
The American submarine force consists entirely of nuclear-powered boats whereas many competing forces field lower-cost, less-complicated diesel-electric types. The U.S. Navy also has the largest underwater force in the world with North Korea coming in a close second. However it should be noted that the North Korean force is made up largely of coastal-minded attack submarines with limited capabilities when compared to larger, nuclear-powered forms fielded by major powers. The top three major submarine powers, therefore, become the United States, China and Russia who command considerable military budgets to keep their expensive submarine forces afloat.
For the purposes of the GFP ranking, landlocked nations are NOT penalized for the lack of a standing naval force though nations with a standing navy ARE penalized for lacking certain naval assets (like submarines). This GFP listing does not make a distinction between nuclear-powered and diesel-electric types nor does it take into account submarine type (attack, nuclear-capable, etc...), design age, or construction / unit quality / training. Submarines strength data through 2017.
Sources: CIA.gov, CIA World Factbook, wikipedia.com, public domain print and media sources and user contributions. Some values may be estimated when official sources are lacking.