Maulana Amjad Ali Azmi Al-Qadri (Hanffi) writes in "Bahr e Shariat" Part 15, Vol. 2: There is difference of opinion on shrimps being a fish or not and based on it, is the difference of opinion on it being lawful or unlawful. Apparently it does not look like a fish. It rather looks like a worm. So one should avoid it. ("Radul Mukhtar") [PS: "Rad-ul-Mukhtar is written by Syed Mohammed Amin Ash-Shaheerba Bin Abideen Shammi, may Allah's mercy be upon him (died 1253H.)]
Following is abstracted form "Sufficient Provision for Seekers of the Path of Truth" by Sheikh Abd' Al-Qadir Al-Jilani (Translation: Muhtar Holland) Vol. 2: In the words of Bakr ibn 'Ubaidillah (may Allah bestow His mercy upon him): "A man can not be truly pious [taqi] until he has come to be pious in a way he approaches the dining room.... According to Shahr ibn Hawshab (may Allah bestow His mercy upon him): "The truly devout person [muttaqi] is one who is ready to abstain from something that is quite harmless in itself, as a precaution against slipping over into something else that could be harmful" . According to Abu Yazid Bastami (may Allah bestow His mercy upon him), true devotion [taqwa] is: "Cautious avoidance [tawarru'] of all things that are of doubtful legality [shubuhat] .
Shrimps (prawns) can also live outside water and no one calls it a fish in regular day to day language. But since the cautious scholars of the past have mentioned the discussions and given no judgment it is up to every individual to decide.
(prawns) ARE A DOUBTFUL LEGALITY!