Making a huge concave object that does not displace water when its
   volume is measured.
   It has a pump and it sucks up the water to counter the attempts to
   measure its volume.
   It is also camouflaged to make it hard to measure its volume visually.
   It has an array of particle emitters and it can dodge, so that if one
   attempts to measure its volume by bouncing things off it, it can also
   try to foil this attempt.