Are prawns Halal? I heard it is undesirable to eat them, but I'm not sure.

Praise be to Allah, the Lord of the Worlds; and may His blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.

Prawns are among the marine animals; they are a kind of fish, so eating them is lawful. There is no reason to consider them undesirable since all things are permissible except those, which are prohibited by a sound text of Shariah.
Allah Says: {Lawful to you is (the pursuit of) watergame and its use for food - for the benefit of yourselves and those who travel,…} [5: 96].
Whoever says that prawns are undesirable should provide evidence.
Allah knows best.