Make the arrow move through the water using washing up liquid.
Arrow templates printed on card, tin foil, paddling pool or large bowl of water, washing up liquid.
Get the Beavers to cut out the arrow templates (one arrow per Beaver). They need to cover the arrows in tin foil (to make water proof). You can also make a small boat but ensure that there is a v-shape at the rear of the boat and that this is level with the water. Could also add sails with paper, cocktail sticks and blue tack (ensure that it floats though). Ensure that the arrow is pointing in the direction that you want it to go and put a drop of washing up liquid at the edge of the v in the middle, make sure that the washing up liquid can get to the water (this displaces the tension in the water which propels the arrow. Watch it move along (need to be quick else you might miss it). Beavers enjoyed doing this and had to keep remaking new ones. Might be worth laminating them (this hasn't been tested to see if it works - should though as it is the surface tension of the water which propels it along). Get the Beavers to predict what will happen when the liquid is added and what happened afterwards.