why is there no TPL ROVs yet
The Blufin 21 platform can be equipped with a PL, no problem. There is at least one from the US Navy on Ocean Shield.
Currently, it can be also be outfitted with sidescan, magnetometer, and video...it is completely autonomous...
EDIT: You program the path of the fish, and to surface at intervals to charge and send data. You can program it to surface at contact as well.