Israel's attack on the aid flotilla was "an act of illegal warfare"

The Legal Position on the Israeli Attack

...

A word on the legal position, which is very plain. To attack a
foreign flagged vessel in international waters is illegal. It is not
piracy, as the Israeli vessels carried a military commission. It is
rather an act of illegal warfare.

Because the incident took place on the high seas does not mean
however that international law is the only applicable law. The Law of
the Sea is quite plain that, when an incident takes place

on a ship on the high seas (outside anybody’s territorial waters) the
applicable law is that of the flag state of the ship on which the
incident occurred. In legal terms, the Turkish ship was Turkish
territory.

There are therefore two clear legal possibilities.

Possibility one is that the Israeli commandos were acting on behalf
of the government of Israel in killing the activists on the ships. In
that case Israel is in a position of war with Turkey, and the act falls
under international jurisdiction as a war crime.

Possibility two is that, if the killings were not authorised Israeli
military action, they were acts of murder under Turkish jurisdiction. If
Israel does not consider itself in a position of war with Turkey, then
it must hand over the commandos involved for trial in Turkey under
Turkish law.

In brief, if Israel and Turkey are not at war, then it is Turkish law
which is applicable to what happened on the ship. It is for Turkey, not
Israel, to carry out any inquiry or investigation into events and to
initiate any prosecutions. Israel is obliged to hand over indicted
personnel for prosecution.
...