You must personally kill as many things as possible, eventually the aliens will be driven back.  At least two drydocks must survive for you to win.

Attack any Crabs attacking Drydocks.

Check behind you for Crabs, CONSTANTLY!

The surface Shadows and Scorpions are attacking the drydocks.  Stop them.

I don’t blame you for trying, but so much damage was done before you launched that you don’t have any chance to save all the drydocks.

Save the Drydocks!