A barrier for small boats has a plurality of flotation modules on lines extending across a waterway. The flotation modules are cast from a buoyant concrete mixture of cement, water, beaded forms of expanded polystyrene, and polypropylene fibers that is non-water-absorbing, crushable, not susceptible to failure in shear, and has a density lighter than water. The mixture can have cement with the beads in a ratio of about 1:3.5 by volume, the water with the cement in a ratio of about 0.5 (1:2) by weight and the fibers with the cement in a ratio of about 1:141 by weight. Lines pass thru the flotation modules and slip upon impact of a flotation module by a speeding boat, and the buoyant concrete mixture crushes to absorb some energy of the impact. The barrier is relatively low cost, can be made next to a waterway from readily available materials by unskilled workers and is deployed in a minimum amount of time.