The default is that all creatures of the sea are lawful, living or dead, based on Allah's words, "Lawful to you is game from the sea and its food" (Quran 5:96), and the Prophet ﷺ said of the sea, "Its water is purifying and its dead animals are lawful." The companions ate a huge whale-like fish (al-ʿanbar) they found dead on the shore, and the Prophet ﷺ approved it, showing that even sea-carrion is permitted. So fish, shrimp, crab, lobster and the like are halal by consensus of the majority. The only real ikhtilāf concerns amphibious creatures such as the frog (which the Prophet ﷺ forbade killing, so most scholars prohibit eating it) and the crocodile, so caution there is best.
Evidence — the daleel
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Informational, not a personal fatwa. Consult a qualified scholar for rulings on your situation.