As far as the UN are concerned the blockade has been declared legal, and it was part of the remit of the panel that was set up to decide on ALL FACTORS regarding the boarding of the Mavi Marmara.
The conlusions of the panel are
The Panel finds:
i. The events of 31 May 2010 should never have taken place as they did and
strenuous efforts should be made to prevent the occurrence of such incidents
in the future.
ii. 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.
iii. The flotilla was a non-governmental endeavour, involving vessels and
participants from a number of countries.
iv. Although people are entitled to express their political views, the flotilla acted
recklessly in attempting to breach the naval blockade. The majority of the
flotilla participants had no violent intentions, but there exist serious questions
about the conduct, true nature and objectives of the flotilla organizers,
particularly IHH. The actions of the flotilla needlessly carried the potential
for escalation.
v. The incident and its outcomes were not intended by either Turkey or Israel.
Both States took steps in an attempt to ensure that events did not occur in a
manner that endangered individuals’ lives and international peace and
security. Turkish officials also approached the organizers of the flotilla with
the intention of persuading them to change course if necessary and avoid an
encounter with Israeli forces. But more could have been done to warn the
flotilla participants of the potential risks involved and to dissuade them from
their actions.
vi. 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;
For more read here
http://www.un.org/News/dh/infocus/middle_east/Gaza_Flotilla_Panel_Report.pdf