Why,yes, thank you for asking. Israel's actions towards the Gaza Armada were legal under International Law.
San Remo Manual on International Law Applicable to Armed Conflicts at Sea, 12 June 1994
PART III - BASIC RULES AND TARGET DISCRIMINATION
SECTION V : NEUTRAL MERCHANT VESSELS AND CIVIL AIRCRAFT
Neutral merchant vessels
67. Merchant vessels flying the flag of neutral States may not be attacked unless they:
(a) are believed on reasonable grounds to be carrying contraband or breaching a blockade, and after prior warning they intentionally and clearly refuse to stop, or intentionally and clearly resist visit, search or capture;
(b) engage in belligerent acts on behalf of the enemy;
(c) act as auxiliaries to the enemy s armed forces;
(d) are incorporated into or assist the enemy s intelligence system;
(e) sail under convoy of enemy warships or military aircraft; or
(f) otherwise make an effective contribution to the enemy s military action, e.g., by carrying military materials, and it is not feasible for the attacking forces to first place passengers and crew in a place of safety. Unless circumstances do not permit, they are to be given a warning, so that they can re-route, off-load, or take other precautions.
A different interpretation here:
Paragraph 47 lists ships that are exempt from attack.
(c) vessels granted safe conduct by agreement between the belligerent parties including:
(ii) vessels engaged in humanitarian missions, including vessels carrying supplies indispensable to the survival of the civilian population, and vessels engaged in relief actions and rescue operations;
The flotilla does not meet the minimum requirement because it was not granted permission by the other party of the blockade, Israel.
Even if it had been, the next paragraph mentions a major exception:
48. Vessels listed in paragraph 47 are exempt from attack only if they: (a) are innocently employed in their normal role;
(b) submit to identification and inspection when required; and
(c) do not intentionally hamper the movement of combatants and obey orders to stop or move out of the way when required.