Give it an old google “In 2015 the State of Palestine became a party to the United Nations Convention on the Law of the Sea (UNCLOS).
Under UNCLOS, coastal states like Palestine can claim sovereign rights over waters extending up to 12 nautical miles from their coast (the territorial sea) and resource rights over an Exclusive Economic Zone (EEZ) that can extend up to 200 nautical miles.
Following its accession, Palestine filed declarations with the UN defining its maritime zones off the Gaza coast.”
So it is in fact Palestinian water - Israel are breaching international law and stealing what does not belong to them, as per usual.
Also a cambridge article on it:
https://www.cambridge.org/core/journals/leiden-journal-of-international-law/article/gaza-marine-the-facts-and-the-law/C029A409DC49A37CA47753C48AED5252