If the students touch the water, make sure they wash their hands afterwards and warn them to not touch anything until they do. This could help reduce any bacteria being spread. Another option is to just have them dry their hands off and to have some antibacterial gel for them to put on their hands. There are differences between standing bodies of water and moving bodies of water as you know. Those differences allow for different things or insects to grow in them.
Some insects live in different types of water because of the type food they eat.
Share with them how you can tell if the water is polluted or not by what lives in it. If there are mosquito larvae, snails and worms in it, it is polluted. If there are Stone flies, Mayflies and Riffle beetles in your water, chances are it is not polluted. Use the pictures of the aquatic insects to help the children identify what they saw. After they have identified the different insects, ask them where they saw each one.