The first submarines (and up to 1944 year) most of the time were on the surface. However, the development of technology has allowed for a long time not to float to the surface of the water, not only the nuclear submarines, but also diesel-electric ones.
Modern submarines are divided by type of power plant, purpose and type of weapons. Strategic submarines are armed with intercontinental ballistic missiles and always have an atomic power plant. Diesel-electric submarines, due to their relatively modest size, can only carry compact rocket weapons.