Its a mystery who they were;
The Sea-peoples must have been a people (pirates/raiders) with whom the Egyptians were completely unfamiliar with otherwise they would have specified who they were; The Egyptians also referred to them as "from the foreign lands"; But the Sea-peoples were most def. a multitude of various peoples as is recorded by there many confederation unions of various peoples;
We are informed by the Medinet Habu (lines 16-17) that before the Sea-peoples came to Egypt they raided all Anatolia (Arzawa/Hatti/Cilicia), Karkemis and Alashiya (Cyprus) which most prob. brought about the Hittite empire collapse; And it is recorded from Tablets (letters) of Ugarit (Rap'anu Archive) that many cities including Ugarit were destroyed by the Sea-peoples; The Sea-peoples must have been according to their armament (and deeds) very skilled soldiers and raiders - for also the Sherden/Shardana were used (Kadesh/RamessesII) as elite mercenaries of the Egyptian Pharaohs; It might be possible that after the 10 year long Trojan-war the Sea-peoples were simply mercenaries and allies of Troja and after its burning raiding and providing for themselves; I believe the bulk of the Sea-peoples were Pelasgians/Maeonians of the Aegean islands and western Anatolia (allies of Troja / Iliad);
A part of the Sea-peoples most prob. later was seeking refuge in Sardinia for the Phoenicians recorded (Nora-stone ~800BC) that Sardinia is the island of the SHRDN (Sherden) and judging by the early 1st mil BC Bronze-age Nuragic figures the resemblance in armament (swords/helmets/shields) is striking to the Sherden who clashed with Egypt in the 12th cen BC;