Density of limestone is about 2.5, so the volume of floats would be about 2.5 times the blocks.
Tied together, a 1m³ block of limestone and a volume of 1.5m³ of floats displace 2.5m³ of water. Assuming the mass of the floats themselves isn't much then already that would lift 2.5 tons off from the bottom of the water pool.