Water play must be one of the earliest forms of leisure activity. You can imagine bands of hunter-gatherers resting along the edges of watercourses to drink and ending up flicking each other with water.
Babies love to splash in their baths; toddlers run fearlessly into pools; young children are amazed by waves. As they mature, children learn to channel much of their early fascination with water into sport: swimming, boating, waterskiing.
We take a fundamental delight in making water do what we want. From water trays in nursery and Reception classes to interactive science shows, making your own dams, canals, rivers and whirlpools is popular, both stimulating and soothing at the same time. Teachers and parents may groan at clearing up the mess, but children beam with satisfaction.
Out of school, children can be more boisterous, enjoying water bombs and water pistols. They may seem anti-social, but there is a lot of basic science in a water bomb.
Try experiments in how much you can fill a balloon with air and how much with water; how thick the skin of the water bomb should be; how far the water disperses on impact; what happens if a balloon bursts; what a slow leak means for water or air. Mass, density, volume, capacity, force: hre they are, splosh on the floor.
Water pistols, particularly the pressurised ones, may seem to have even less to redeem them. They are modelled on guns, after all, and squirt uncomfortably strong jets. But try making one of your own. You will need to construct a simple siphon and pump. Then when you start to practise aiming and firing, you will quickly discover that as the water pressure weakens, so the range of fire drops: the laws of motion, friction and inertia apply.
It would be interesting to see if any of the "supersoakers" on the market actually live up to their packaging descriptions. I have yet to find one that shoots anything like its claimed 1,000 metres, or even 30 metres (the cheapest version).
Water has many amazing qualities. Translucent, more or less odourfree, potable, it also dissolves substances.
In class, you can make beautiful wallpaper-type graphics by dissolving paint colours in water and using the different settings on a plant mister to create interweaving fans of colours on large sheets of paper. Perhaps you could make a new background for your class displays or covers for books.
Instead of telling your class to surrender their water pistols, get them out and get them working for you.