Cationic polymers are a group of chemicals that share some common characteristics. As polymers, they are all big chemicals. Under environment conditions, that are positively charged (or cationic), which means they can have toxic effects on aquatic life. They are generally also man-made, and man-made polymers are usually not readily biodegradable.
The toxicity of a cationic polymer is related to how many parts of the chemical have a positive charge, relative to the overall size of the chemical. If the chemical is smaller and lots of parts have positive charges, it will be more toxic. Where the parts with positive charges are more spread out, it will be less toxic.
In the environment, chemicals that are positively charged can bind to particles of dirt and sediment, as well as dissolved carbon. This can reduce the potential for the chemical to cause toxic effects, because organisms are less likely to be exposed to it. The impact of this can vary, as each environment is different. Few studies have been done to confirm what an average effect is, but a standard assumption is that the actual toxicity will be a level lower than otherwise expected (i.e. toxic instead of very toxic, or harmful instead of toxic).