A flood defense or water-retaining device 8 comprises a hollow body of an elastomeric material having formations 30, 34 at opposite ends for sealing engagement with complementary formations on adjoining flood defense devices and having complementary formations 22, 24 on upper and lower sides for sealing engagement with complementary formations on lower and upper sides respectively of any similar devices disposed respectively above and below the first-mentioned device. The device 8 has at least one filling opening 40 through which water can be passed into the interior of the device to fill the same.