Things like temperature and salinity measurements are totally standard oceanography/climatology/general civilian science of various degrees of applied-ness; and the equipment to make those measurements(not necessarily the top shelf stuff, which might be exclusive to those with bigger budgets) is fairly prosaic. Any university or research institution with a decent oceonography program should be able to hook you up; and scientific publications aren’t considered all hush-hush.
However, salinity and temperature(along with seabed geometry and assorted other variables, apparently including the swim bladders of local fish, if they are present in sufficient numbers, which I find inordinately amusing) affect the acoustic properties of seawater, which means that anyone operating sonar(active or passive) will have a better time making sense of the data if they also have accurate and current information on the acoustic properties of the water they are working in.
I have no special knowledge of what this UUV is capable of, or what it was doing; but it wouldn’t be technologically infeasible for it to be a more or less off-the-shelf, unremarkable, research grade device; but gathering data as part of a navy effort to ensure optimal day-ruining for Chinese naval assets in the area.
