Maybe you can include links to what you are referring to so we can get an idea what you mean.
I’m not an expert on aquatic remote sensing, but I suppose that there are many approaches on this.
you can apply equations to rasters by right-clicking them and select “Band Maths”. I’m not aware of a linear function that retrieves turbiditiy from a reflectance image, sorry.
Do you have an example for a contour diagram? I’m afraid I don’t have an idea how this could look for a multi-spectral image. Maybe you can refer to an external image which shows something similar.
For most empirical models, it is better to use the reflectance because it is calibrated.
The maps you linked show some sort of classified thematic map, but I don’t understand the language. Please have a look at SNAP’s “Thematic Land” and “Thematic Water” processing toolboxes and check if some of them come closer to what you need.
you can either use scatter plot to plot reflectance against the latitude (tie-point-grid) or use the profile tool to plot the reflectance along a line.
You can create a logical band maths expression (latitude > 35 and latitude < 40)
This creates a mask band for all pixels which fulfill this criterion. You can then use this mask in the plot view. I haven’t tried it but it should work.