The phrase "a sea of tranquility" often refers to a sense of calmness and peace, evoking imagery of a serene body of water. It can also relate to the Moon's "Sea of Tranquility" (Mare Tranquillitatis), a large lunar mare that sits within the Moon's surface, famously known as the landing site of Apollo 11 in 1969 where humans first walked on the lunar surface.
In a metaphorical sense, a "sea of tranquility" represents a state of mind where chaos