Excerpts from the UN Report which can be read in PDF format here:
Report of the Secretary-General's Panel of Inquiry on the 31 May 2010 Flotilla Incident

humanitarian missions must respect the
security arrangements put in place by Israel. They must seek prior approval from Israel
and make the necessary arrangements with it. This includes meeting certain conditions
such as permitting Israel to search the humanitarian vessels in question.

The fundamental principle of the freedom of navigation on the high seas is
subject to only certain limited exceptions under international law. Israel faces a real
threat to its security from militant groups in Gaza. The naval blockade was
imposed as a legitimate security measure in order to prevent weapons from entering
Gaza by sea and its implementation complied with the requirements of international
law.

For Israel to maintain the blockade it had to be effective, so it must be enforced.
That is a clear legal requirement for a blockade.344 Such enforcement may take place on
the high seas and may be conducted by force if a vessel resists. To this point in the
analysis no difficulty arises. But the subsequent steps taken raise serious questions as to
whether the enforcement was executed appropriately in the circumstances

Israel's decision to board the vessels with such substantial force at a great
distance from the blockade zone and with no final warning immediately prior to the
boarding was excessive and unreasonable:
a. Non-violent options should have been used in the first instance. In
particular, clear prior warning that the vessels were to be boarded and a
demonstration of dissuading force should have been given to avoid the type
of confrontation that occurred;
b. The operation should have reassessed its options when the resistance to the
initial boarding attempt became apparent so as to minimize casualties.

133. Israeli Defense Forces personnel faced significant, organized and violent
resistance from a group of passengers when they boarded the Mavi Marmara requiring them to use force for their own protection. Three soldiers were captured,
mistreated, and placed at risk by those passengers. Several others were wounded.
134. The loss of life and injuries resulting from the use of force by Israeli forces
during the take-over of the Mavi Marmara was unacceptable. Nine passengers were
killed and many others seriously wounded by Israeli forces. No satisfactory
explanation has been provided to the Panel by Israel for any of the nine deaths.
Forensic evidence showing that most of the deceased were shot multiple times,
including in the back, or at close range has not been adequately accounted for in the
material presented by Israel.

The blockading power is entitled to board a neutral merchant vessel if there are
reasonable grounds to suspect that it is breaching a blockade. The blockading power has
the right to visit and search the vessel and to capture it if found in breach of a blockade.
Breach could occur outside the blockade zone, including on the high seas where there is
evidence of the vessel's intention. If there is clear resistance to the interception or
capture, the blockading power may attack the vessel, after giving a prior warning. The
level of force used to enforce the above-mentioned rights must be proportionate; in
particular, it must be limited to the level necessary to achieve the military objective.