Salamun Alaikum,
When the Quran speaks about dead / carrion (animals)
'maytatu' (5:3), we need to understand it in the context of the animals that we are allowed to eat and whose life has to be ended in a particular lawful way (thakaytum) (5:3).
This is dealt with in the very first verse of the chapter 5:1,
"uhillat lakum bahimatul anaam" (lawful to you 'grazing livestock'). (5:1).
5:3 'maytatu' (dead) is not talking about 'fish' or the 'catch of the sea / rivers'. It is talking about the type of animals mentioned in 5:1 (grazing livestock).
The catch of the sea (fish etc) are dealt with in another verse by the phrase
"uhilla lakum saydul bahr wa ta'amuhu mata'an" (Lawful to you is the game of the sea and its food as provision)
The catch of the sea does not require the ending of its life to be performed in a particular lawful way
(thakaytum) as do the 'animals' (grazing livestock bahimatul anaam) in 5:3.
There is a difference.
The life of the animal (5:1, 5:3) has to be ended by a lawful process required by the Quran
(thakaytum). The lives of the creatures of the sea are usually ended when you take them out of their environment (sea / river). There is no further process required to make them lawful according to the Quran (5:96).
I hope that helps, God willing.
Joseph.
Related Articles:THAKAYTUMhttp://quransmessage.com/articles/thakaytum%20FM3.htmFOOD PERMISSIBILITY AND PROHIBITIONShttp://quransmessage.com/articles/food%20restrictions%20FM3.htm