  As an emerging channel resource for modern optics, big data, internet traffic
and quantum technologies, twisted photons carrying orbital angular momentum
(OAM) have been extended their applicable boundary in different media, such as
optical fiber and atmosphere. Due to the extreme condition of loss and
pressure, underwater transmission of twisted photons has not been well
investigated yet. Especially, single-photon tests were all limited at a level
of a few meters, and it is in practice unclear what will happen for longer
transmission distances. Here we experimentally demonstrate the transmission of
single-photon twisted light over an underwater channel up to 55 meters, which
reach a distance allowing potential real applications. For different order OAM
states and their superposition, a good preservation of modal structure and
topological charge are observed. Our results for the first time reveal the real
transmission performance of twisted photons in a long-distance regime,
representing a step further towards OAM-based underwater quantum communication.
