Passengers and the teen can be heard laughing as he heaves himself back on board after dangling from the ship for a few moments.
In a hair-raising moment, the young daredevil briefly appears to struggle to lift his weight back onto the lower deck as another a passenger says: "Are you alright?"
The clip was posted on Twitter by Anthony Rosale, who wrote: "Craziest dude I met on my cruise".
It was reportedly filmed on the Carnival Liberty, a 110,000-tonne cruise liner, while at sea off the Texas coast on November 19.
The stunt was condemned as "incredibly dangerous" by fellow passengers and the ship's owners.
Buffy Halbrooks Augustine told Click 2 Houston: "This literally made my heart beat super fast and made me nervous. Kids just don't understand that they are not invincible."
"He thought he was having fun and never considered the possibilities."
Another passenger, who gave his name as Marcus K, added: "It's incredibly dangerous. He would have been sucked under immediately."
In a statement, Carnival Cruise Line said: "We recently became aware of the video and are investigating. We strongly condemn any such behavior by our guests that puts themselves or others at risk."
A 33-year-old woman is presumed to have died plunging overboard from the same ship in May this year.
Mum-of-four Samantha Broberg was seen falling backwards into the water as she sat on a railing at 2am.
The US Coast Guard spent 20 hours scouring 4,300 square miles of the Gulf of Mexico before calling off the search.
Ten things not to do on a cruise
Ten things not to do on a cruise
This is really frowned upon – and some cruise lines have banned 'chair hogging' completely.
You may happy exploring a destination on your own – particularly if it's somewhere you've visited before. But there are usually some amazing experiences on offer, from jeep safaris in Jamaica to dolphin exploration in Dominica and a classical concert in St Petersburg, so it's definitely worth checking them out.
Unless you can afford to book an extra cabin for your luggage (and we have heard of some very rich passengers doing this), average staterooms don't have masses of storage space. So just pack a capsule wardrobe – although it's always worth taking a couple of swimsuits so one can dry while you wear the other one.
Some passengers decide that because they've already been to a place they don't need to get off at a port – but there is always something new to discover, particularly if your ship offers 'destination immersion' shore experiences, or even just to get a change of scene. Many cruises have one (maybe more) days at sea on a cruise, so plenty of time to hang around the ship then.
OK, if you have really strong willpower, you *may* be able to avoid all the gourmet food (and there are usually lots of salads and healthy options available). But with so much excellent cuisine on offer it would be a shame to miss out on more the more indulgent creations from the likes of Marco Pierre White and patissier Eric Lanlard.
Of course holidaying with your family is a fantastic opportunity to spend quality time together – but cruise ships have amazing kids' clubs, with every possible whim catered for. Not only does it give your children a chance to make new friends and try different experiences, but it will give you a little alone time too. For example, on Oriana, Aurora and Azura, the kids' clubs are open until late – ideal if you want to give the kids an early supper and then enjoy dinner a deux knowing they're in safe hands.
It's compulsory. Plus, you'll also discover how to put on a life jacket (not as easy as it looks) and what a muster station is.
Joining the other passengers on deck as your ship pulls away from the dock is something very special and adds to that 'golden age of cruise' feel. There will usually be cocktails on offer so you can toast the start of your holiday, too (although check before you take one, as some cruise lines do charge for these).
The cabin staff work incredibly hard, and are often away from their families for months on end – so show your appreciation. Some cruise packages include tips – check what's included in yours and if you want to leave a little extra in an envelope, it will be gratefully received.
You can do this up to a point – it is your holiday, after all. But if you want to make sure your restaurant seating is at the right time or you can have that fabulous spa treatment (alfresco massage, anyone?), you'll need to do a little planning. Don't feel you have to do everything though – if you overschedule you'll just feel pressured.