What is the morphological differences between a grasshopper and a prawn?

Hugely different organisms.

External obvious differences:

Prawns have swimmerets (pleopods), a tailfan (uropods flanking telson), rostrum, claws (chelated pereiopods), stalked eyes, long antennae, antennules.

Grasshoppers have wings, large hind legs (femurs?), short antennae, unstalked eyes.

Ignore morphology, if it's living underwater it isn't a grasshopper. ;)