I am a citizen of India who wants to travel (using a commercial private ship) to a point in sea/ocean which isn't under control of a country. I presume I won't require a Visa. But, would I be allowed to leave the shore legally without Visa? Which documents do I need? And, what are the legal formalities required?
Nothing and nobody can justifiably stop you heading to international waters. ( The freedom of the seas)
However, that's only half the story: afterwards, you will have to enter somebody's territorial waters (unless you plan to stay at sea forever to avoid taxes; some very rich people have plans to do just that). When you do, you will need visas, customs clearance, and immigration checks, just like anybody else: if you are not admitted, it is your carrier's responsibility to take you back. Because of this last point, many airlines will not allow you to depart unless your arrival clearances are in order, and it may be wise to check with your shipping line.
If you plan just to re-enter India, it's difficult to see how you can be prevented: by definition, you're not importing anything, and all Immigration could possibly do would be return you to your point of origin, i.e. India. But logic has never stopped bureaucracy, so I would recommend contacting the Bureau of Immigration before you leave to ask how they suggest you should handle it.
Update: There is a long tradition in England of taking a day trip to France purely for the sake of saving money on the duty-free goods you can buy while not subject to any national taxation. Some years ago, a ferry company took this to its logical conclusion, of sailing out to international waters, going in circles for a few hours while the shops did good business, and heading back to England. This is no longer in operation, alas, but as I remember you did not need a passport but definitely did need to clear Customs on return (though in practice the officers often just gave up the task of checking that several hundred merry day-trippers had not miscalculated their allowances).
According to the Freedom Of The Seas principle you can travel to the areas which isn't under any control of a country, that is called international waters. Just make sure that you do not cross the sea borders of any country. This happens a lot in some areas like the Arabian Gulf where Arab ships cross to the Iranian borders by mistake and vice versa.