Wa Alaykum Salam Warahmatullah,
According to Imam Abu Hanifah (R.A) all marine animals with the exception of fish are Haraam. Lobsters and crabs will therefore not be permitted according to Imam Abu Hanifah (R.A).
Concerning prawns, there is difference of opinion among our Noble Ulama (R.A) as to whether it is a fish or not. Some Ulama are of the opinion that prawns are fish and thus Halaal while other Ulama have said that it is not a fish.
Hazrat Mufti Syed Abdur-Raheem Saheb Lajpuri (R.A) is of the opinion that prawns are fish. He has quoted a number of proofs to substantiate his view. (Fatawaa Rahimiyyah, Page 258, Vol. 7)
Hazrat Hakeemul Ummat Moulana Ashraf Ali Thanwi (R.A) was also of the opinion that prawns are fish. (Imdadul Fatawaa, Page 101 Vol. 4)
According to Imam Shafi'ee all sea creatures with the exception of a frog are Halaal.
Imam Malik (R.A) is of the opinion that all sea creatures with the exception of sea-pig are Halaal.
(Dars-e-Tirmidhi - Hazrat Mufti M. Taqi Usmani Sb. Page 279, Vol 1)
Allah SWT Knows Best
(Mufti) Abdullah Patel