Higher side lobe levels in linear array beam pattern
There are several practical items that may cause reduced beamforming performance and lead the higher sidelobes. You haven't said how high exactly these sidelobes are, nor what levels you expected.
Some common areas to investigate further:


Array element spacing. To prevent the presence of grating lobes your element spacing needs to be less than $\frac{\lambda}{2}$. You haven't indicated the bandwidth of your signal, so I'll use the
400 KHz you gave and assume a speed of sound to be 1500 m/s. Then your spacing needs to be less than $\frac{1500}{2\cdot 400\cdot10^3}=1.875$ mm. Of course you should use the highest frequency of signal of interest for this calculation.
I'm assuming that you are using IQ sampling. Phase errors in the IQ demodulation signal or slew in the clocking (synchronization) across the 80 channels may degrade the performance. At this high frequency you will need very small tolerances to limit the effect of these phase errors. 
Have you considered the effect of Multipath propagation? The effect of the signals bouncing off the side of the tank or the water surface can lead to signal levels with a higher intensity (constructive interference) than the level of your direct path.