121 CG is Philadelphia's or should I say sector Delaware Bay's primary operating frequency.
Sometimes it is in the clear, sometimes one side is encrypted. I imagine there are times both sides are encrypted but I wouldn't know lol.
This has been the Philadelphia areas Coast Guard frequency for a while now but they do use 81a.
Keep in mind all the regulars like 16, 22, 21,23,13, 83 are also used. In the Philly area this is all Delaware River traffic. All Ships on the Delaware River must monitor 16 + 13. Coast Guard is 121 CG... we also hear UHF Coast Guard on 409 CG, that's UHF digital.