Seafood is any form of sea life regarded as food by humans, prominently including fish and shellfish.
Yes, it is permissible to eat the skin of fish which has scales on it, and was taken alive from water, if it does not harm your health.
Eating poisonous food is not halal. The idea is that eating seafood is a blessing we are allowed to partake in (with the appropriate guidelines) without it being extremely complex and that it is one we may make use of and thank Allah for.
Not everything that comes from the ocean is halal unlike what some other sects might claim.
In our jurisprudence for example, one cannot eat fish that doesn’t have scales. Jurists also differ as to the amount of scales which must be present on the fish. For example, some fuqaha say that it is only allowed to eat a fish if MOST of the body has scales, and others say that even a tiny amount of scales is sufficient. It should be noted that prawns are also halal.
The above ruling are based on the ahadith such as the following:
Imam Al-Baqir (peace be upon him) said:
“Eat any fish that has scales, and do not eat what does not have scales."
[Al-Kulayni, Al-Kafi, Vol.6, p. 219]
May Allah grant you success
Haraam sea food is not allowed to be eaten but it is not Najis, that is why, Halal food cooked in utensils that were used for Haraam seafood is allowed.
This is only for the non Halal seafood because it is not Najis, otherwise, every non Halal food like meat, chicken etc is Najis and utensils used in cooking it become Najis and must be made Taahir before we can use it in cooking.
It is good to mention that non Halal seafood includes every seafood except scaled fishes and prawns.