One reason India is hustling to develop SSBNs (ballistic missile carrying sub) is because of their problems building enough nuclear weapons and reliable ballistic missiles to deliver the nukes to Chinese and Pakistani targets. While land based nuclear weapons (delivered by aircraft of ballistic missile) can, in theory, be wiped out in a surprise attack, those carried on an SSBN are hidden and available for a retaliatory strike. In effect SSBNs exist to eliminate the ability of a foe making a surprise attack against your land-based nukes.
Currently, India and Pakistan each have about a hundred nuclear weapons, while China has at least 250. The only one of the three that takes nuclear warfare seriously is Pakistan, which built nuclear weapons in the belief that India was planning to conquer Pakistan. This was absurd but the belief was heavily promoted by the Pakistani military to keep the money coming (and their economic and political privileges intact). Many Pakistani politicians promoted the "aggressive neighbor" angle as well. But in fact it was Pakistan that had been attacking India since the two nations were created in 1947. Every time India pushed back Pakistan lost. Now, in addition to its delusions, Pakistan has nukes and much more inclination (than India or ally China) to use them.
India has always had problems developing military technology. The list of failed and much-delayed projects is long. Unfortunately nukes, missiles, and SSBNs are all very high tech. The SSBNs have been having problems and are not yet at sea scaring anyone.
It was in 2009, that India launched its first nuclear submarine, the INS Arihant (Destroyer of Enemies). This came after over a decade of planning and construction. What was not revealed at the times was that the Arihant was launched without its nuclear reactor, which was not ready until the following year.
The Arihant was launched when it was because work on the sub had been going on for more than a decade and it was becoming embarrassing to have nothing to show for all that effort. The first of six Arihants was supposed to enter service this year as an SSBN. The 6,000 ton Arihant has four vertical launch tubes, which can carry twelve (three per launch tune) smaller K-15 missiles (with a range of 1,900 kilometers) or four larger K-4 (based on the Agni III) missiles, each with a range of 3,500 kilometers. The K-4 is still in development. Two more Arihants are under construction.
The Arihant is based on the Russian Charlie II sub, which it resembles. A leased Russian Akula II nuclear sub will basically serve as a training boat for India's new nuclear submarine force. Russia retired all its Charlie class subs in the early 1990s. India leased one from 1988-91, and gained a great deal of familiarity with it. The Charlie class had eight launch tubes, outside the pressure hull, for anti-ship missiles.