  Atmospheric free space and fiber have been widely exploited as the channels
for quantum communication, and have enabled inter-continent and inter-city
applications. Air-sea free-space channel, being capable of linking the
satellite-based quantum resource and underwater vehicle, has now become the
last piece of the puzzle in building global quantum communication network.
However, long-distance quantum communication penetrating water up to tens to
hundreds of meters is extremely challenging due to the inevitable high loss.
Here, we present an experimental demonstration of underwater decoy-state
quantum key distribution against high loss, meanwhile keep a low quantum bit
error rate less than 2.5% for different distances. By directly modulating
blue-green lasers at a high speed of 50MHz and decoy-state protocol, we are
able to for the first time reach a long-distance quantum key distribution that
is unconditionally secure and can enable real-life air-sea quantum
communication tasks. The demonstrated distance, even in coastal water of Jerlov
types 2C, is up to 30 meters, about one-order improvement over the
proof-in-principle demonstrations in previous experiments, and the channel loss
is equivalent to 345-meter-long clean seawater of Jerlov type I, representing a
key step forward to practical air-sea quantum communication.
