Zippered plastic bags or buckets with lids can transport water and samples to the classroom. Use a microscope to see plankton, algae, etc. Measure acidic or basic water properties. Aquatic organisms have different ranges of pH tolerance. Some ranges are broad, some narrow.
Sea water tends to be basic.
Test for saltwater intrusion, especially when dead oak trees are present. Compare salinity of one body of water with another. Remember, fertilizer runoff can increase after heavy rain. Use a Secchi disk which you can purchase or make. Turbidity is a measure of how clear or cloudy water is.