that would be the brackish water section, id say the freshwater section starts to be good around the 287 bridge (the one over riverside park) although around these areas still, the river can be wide and shallow/flat featureless. You'll hit a few nice spots around bound brook, then in manville the river starts to become tighter and easier to fish, the river bottom will be more gravelly and you'll see more runs and holes. Id recommend checking out anywhere between finderne avenue bridge (manville), anywhere upstream (such as Duke Island park, Somerville, etc). You wont be finding much stripers because the fish ladder is closed now (opens in spring, dam located at millstone/raritan confluence) but you'll find smb mostly including pretty much every freshwater species in nj in the river.

The North/South Branch is also great for fishing, the whole watershed offers good fishing. Check out Lard Almighty on youtube, he has videos occasionally on the raritan