Might the Soviet
US-A (Western designation Radar Ocean Reconnaissance Satellite/RORSAT) series satellites and the
TOPAZ reactor powered
Kosmos 1818 and
Kosmos 1867 satellites have been used in that role, at least with the later units? The US-A series orbited Earth very closely and getting any useful information from a high altitude naval radar would have required development of
look-down capability to eliminate clutter.
The United States didn't deploy synthetic aperture radar satellites until many years after the Soviets put theirs up, but the United States and NATO could far more easily ring the ocean with sound detection equipment and early Soviet submarines were notoriously noisy. The Soviets were at a disadvantage in lacking friendly areas to base sound detection equipment out of and having to hunt submarines that were far quieter.
This is getting more into radar physics than nuclear power, but I'm wondering if this is something that the Soviets might have been trying for with their naval reconnaissance satellites.