There are many theories as to who they were. The wiki page has nine hypotheses.
What is the most widely accepted theory on who the ancient Sea Peoples were?
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The sea peoples are in Egyptian sources referred to as "foreign people of the sea". It is in other words a name they use for any sea-faring foreigners that attack Egypt. Some of the sources list the names of these sea peoples, making it clear that it is not one people that is meant.
The theories mentioned on Wikipedia may therefore all be correct at the same time.