It's not my water
In anycase, looking at the pic, I'd expect that without water inside, that you would be able to budge it, quite easily in fact, but the water is creating the weight and is emphasising the power of water, especially down the beach.
Without the water, the weight would not be evenly distributed, meaning if it was top or bottom heavy, you'd still get some leverage and movement.
Fair enough to presume there is actually water inside