Submarines can be powered by nuclear reactors or by diesel engines paired with batteries. Nuclear boats can stay submerged for long periods, travel fast, and operate far from home without refueling.
Diesel-electric boats are generally cheaper and can be very quiet when running on batteries, but they must come up periodically to run their engines, and their range and endurance are more limited.
Navies choose based on missions, geography, and budgets. Debates about the right mix involve cost, industrial capacity, and the kind of operations expected.
