Does anyone know how dirty the Seine was in the last hundred years?

The Seine has been dramatically cleaned up.

Here’s a series of charts that plots two measures of water quality, oxygen (blue) and ammonia (red) content, over the past century and a quarter. High oxygen content is good, high ammonia content is bad. The charts show measurements at various points downstream from Paris (the numbers on the x-axis are kilometers from Paris), and represent data from the low-flow period in summer when conditions are worse.

Nowadays, the conditions for supporting aquatic life are almost always quite good. Bacterial content is still pretty high though, so swimming in the Seine is not advised. In Paris, the Seine is slightly turbid (not surprising for a high-flow river), but generally free from unpleasant odor.

I know there was a big flood of the Seine in Paris in 1911; researching that period might give you some idea of the state of the river then.