Yes I think an experimental submarine is most likely.
It's screw and control surfaces suggest typical conventional submarine performance envelope, maybe a bit quicker because of the sleek bow and tiny 'bump' sail. But would expect a pumpjet if speed really was the objective, unless China cannot build those yet(?). Also note that the lower rudder appears short enough to allow it to bottom-out on the sea floor, hmmm...
<edit: Hood already noted the lower rudder, good stuff! yes. However cannot see other obvious hints of special forces mission except size and seemingly modest or non-existent sonar/torpedoes which point to that type of mission. >
Possibly it is an export model, that much has been speculated by Chinese observers, but seems a bold design. Pakistan might be the customers if export (cannot think of any other likely country), but I don't think it is.