It's a Mola mola, aka a common mola. That is what it is.
According to National Geographic, molas can grow to be as long as 14 feet, and can weigh in at an astounding 5,000 pounds. They are the heaviest bony fish in the world with one of the most unusual statures. Molas are affectionately called ocean sunfish because they prefer temperate and tropical areas, and often surface in summer months to bask in the sun.
This guy was spotted off Brown's Island in Marblehead, Mass., which makes sense given that the fish frequent areas like Cape Cod and Massachusetts Bay.
For more details on these ocean aliens and to learn more about conservation efforts, click here.
Via David Fernandes