That will probably sink easily, but if the bottom of your pond or lake has a hard surface and a lot of boat traffic, it may not stay in place. This spring I plan on fastening 2 or 3 blocks together as the lake here has 60 foot boats and bimbo boatmen or boatwomen. The waves from their boats deflect from the shoreline and have a tendency to push out anything you place on the lake's bottom!