Don't pump oily water from your bilge into the waterway. Well-maintained engines shouldn't leak oil, but check the drip tray under the engine and gearbox regularly. Use biodegradable oils, if possible. Avoid spilling petrol and diesel. If you do, mop it up - don't use detergents.
The toilets on your boat must not discharge sewage into the waterway. There are pump-out facilities for chemical or closed toilet systems at marinas and sanitary stations. Use the minimum amount of chemicals to avoid upsetting the sewage treatment system. If you have a closed toilet system, you may not need to use chemicals at all - so check your manual.
The wastewater outlet from your sink and shower is allowed to flow straight into the waterway. But to help keep the water as healthy as possible, put your cooking waste in the bin, use environmentally friendly detergents and be economical with everything you put down the sink.