The 'Sea People', were simply successive waves of priracy and migration forced upon the eastern Mediterranean by circumstance. More than likely, they were not any one single group of people.
As to their impact and the devastation they may or may not have caused. 'They' were probably not as big of a problem as the famine, drought, and earthquake storms, that set them in motion in the first palce.
"The foreign countries (ie. Sea Peoples) made a conspiracy in their islands, All at once the lands were removed and scattered in the fray. No land could stand before their arms: from Hatti, Qode, Carchemish, Arzawa and Alashiya on, being cut off [ie. destroyed] at one time. A camp was set up in Amurru. They desolated its people, and its land was like that which has never come into being. They were coming forward toward Egypt, while the flame was prepared before them. Their confederation was the Peleset, Tjeker, Shekelesh, Denyen and Weshesh, lands united. They laid their hands upon the land as far as the circuit of the earth, their hearts confident and trusting: "Our plans will succeed!"
As you can see, he says that several Anatolian peoples, including the Hittites were defeated and burned by the invaders. Also, Cyprus (alashiya), Cilicia (Qode) and in Syria (Amurra).
The Sea Peoples, as we call them, which came from land and sea were a confederation of several tribes. There are various theories about who those peoples were.