Question

why is osmoregulation is necessary in aquatic organisms

Answer 1

So that they don't swell up or shrink during the variation in the surrounding water

Answer 2

osmoregulation means maintaining water and ion concentration.
In aquatic animals if it is not maintained the water will come into their body and they will either explode or die of thinning of membranes.
thats why osmoregulation is necessary for their life.