A great way to explore the movement of water with kids is to take a trip to the seashore. Stand at the water's edge, holding hands and keeping your feet in the same spot. Talk about the water as it rolls in, up, over, and around your feet, and then recedes. Will the next wave be bigger or smaller? Will it touch your toes or cover your feet completely?
If you can stay a while, you will see the tide change. If the tide is coming in, dig ditches or "river beds" near the water's edge and watch the water come rushing through. If the tide is going out, mark where the waves come each time and then see what happens.
If a trip to the ocean isn't in sight, try creating a tide in the bathtub. Place some bath toys or homemade boats (made from pieces of Styrofoam) in the empty tub. Put the plug in. Then, turn on the water so it comes out slowly. Watch with your child as the water spreads further and further back into the tub and begins to lift the toys.
What happens when you make the water come out faster or slower? Does that change the way the boats move? What happens to the toys when you drain the tub?