Praise be to Allah.
Islam: Easy to follow
Among the blessings that Allah has bestowed upon us is the fact that He has made our religion easy for us, and has not made it too difficult or unbearable. He has allowed us many things that were forbidden according to previously-revealed laws. Allah says (interpretation of the meaning): “… Allah intends for you ease, and He does not want to make things difficult for you…” [al-Baqarah 2:185]
Is seafood halal?
Hence all kinds of food from the sea are permissible, whether they are plants or animals, alive or dead. Allah says (interpretation of the meaning): “Lawful to you is (the pursuit of) water-game and its use for food – for the benefit of yourselves and those who travel…” [al-Maidah 5:96]. Ibn 'Abbas said: “ Sayduhu (lit. hunting, pursuit) refers to whatever is taken from it alive, and ta’amuhu (lit. its food) means whatever is taken dead.”
Forbidden water animals
There are a few things – certain types of water animals – which some scholars exclude from the permission outlined above. These are:
1- Crocodiles: The correct view is that eating these is not allowed, because they have fangs and live on land – even though they may spend a lot of time in the water – so precedence should be given to the reason for forbidding it (it is a land animal that has fangs).
2- Frogs: It is not permitted to eat them because the Prophet (peace and blessings of Allah be upon him) forbade killing them, as is reported in the hadith of ‘Abd al-Rahman ibn ‘Uthman, who said that the Messenger of Allah (peace and blessings of Allah be upon him) forbade the killing of frogs. (Narrated by Imam Ahmad and Ibn Majah; see also Sahih al-Jami’, 6970).
The rule is that everything which we are forbidden to kill, we are not allowed to eat; if we are allowed to eat it we are allowed to kill it.
Some scholars exclude sea snakes, but the correct view is that as they live nowhere except in the water, we are permitted to eat them, because of the general nature of the ayah (interpretation of the meaning): “Lawful to you is (the pursuit of) water-game and its use for food – for the benefit of yourselves …” [al-Maidah 5:96]
3- Otters and turtles: The correct view is that to be on the safe side, it is permissible to eat them after slaughtering them properly, because they live both on land and in the sea.
Here the rule is that in the case of animals that live both on land and in the sea, the rules concerning land animals should be given precedence, to be on the safe side, so they must be slaughtered properly, except for crabs which do not need to be slaughtered, even though they live both on land and in the sea, because they do not have blood.
Everything that can cause harm is forbidden as food, even if it comes from the sea, because Allah says (interpretation of the meaning): “… And do not kill yourselves (nor kill one another). Surely, Allah is Most Merciful to you.” [al-Nisa 4:29] and: “… and do not throw yourselves into destruction…” [al-Baqarah 2:195]
(See al-Mughni, 11/83; Hashiyah al-Rawd, 7/430; Tafsir Ibn Kathir, 3/197; and Ahkam al-At’imah by al-Fawzan).
For more about halal food issues, please see these answers: 82444 , 1814 , 1938 , 20039 , and 210
And Allah knows best.