The only times you're very likely to find the broadcast networks, per se, on cabin TV's is when you're in a US port city that has local stations affiliated with those networks. In those cases the people in the TV shack can just pick up the stations off-air and feed them to the rooms. But even in those cases they usually only feed one or two, so you can't be sure you'll get the one you want.
At sea and in foreign ports you'll only get the satellite services that the line has contracted for (and that happen to have a solid satellite downlink where you are.) These usually include CNN and/or CNN International, ESPN, and maybe one or two other cable networks. I've sometimes seen Fox Sports Net (which is different from the regular Fox network). Was on the Brilliance of the Seas during the World Series, and they had zero games available, which was disappointing. But they did have some NFL football games.
In addition to that, some lines run DVD's on some channels of "older" network TV shows. These feeds originate on the ship as opposed to being picked up off-air or off-satellite. They seem to be quite partial to the first season of West Wing for some reason. I reckon they do this so that in case they get into a satellite "hole" where reception is bad, at least there will be something on TV besides the port lecture and the cruise director begging you to be generous with your tips.
There is the story about the guy in the sports bar who was getting frustrated because the satellite signal of some big game he was watching was fading in and out. His wife, sitting near him, shared his frustration. "What's the matter with them? Don't they have cable?, she whined."
We just got back from the Christmas sailing on Summit...you'll definitely be able to catch the playoff games-either in the Cinema, the Casino bar or your room depending on where they choose to show it.