Hi @ugurdemirezen,
The only dedicated underwater charging system I’m aware of is in this project, which was able to wirelessly charge at up to 100W.
Beyond that I expect it’s most common to use either entirely topside power through a tether (there are several systems that do this), or a hybrid system that uses power from the topside to supplement the primary power provided by a battery. Note that topside power can still include some degree of autonomy, for example from a boat/buoy with solar panels.
I expect approaches with bare metal contacts in the water would likely suffer from issues with corrosion and/or marine growth.