Israel doesn't have the legal right to stop ships that are carrying humanitarian goods. That also means the ships nor crew nor passengers can be seized and prevented from continuing on to Gaza after the cargo has been inspected.
CONTRABAND OF WAR
Art. 22. The following articles may, without notice (1), be treated as contraband of war, under the
name of absolute contraband:
(1) Arms of all kinds, including arms for sporting purposes, and their distinctive component parts.
(2) Projectiles, charges, and cartridges of all kinds, and their distinctive component parts.
(3) Powder and explosives specially prepared for use in war.
(4) Gun-mountings, limber boxes, limbers, military waggons, field forges, and their distinctive component parts.
(5) Clothing and equipment of a distinctively military character.
(6) All kinds of harness of a distinctively military character.
(7) Saddle, draught, and pack animals suitable for use in war.
( Articles of camp equipment, and their distinctive component parts.
(9) Armour plates.
(10)Warships, including boats, and their distinctive component parts of such a nature that they can only be used on a vessel of war.
(11)Implements and apparatus designed exclusively for the manufacture of munitions of war, for the manufacture or repair of arms, or war material for use on land or sea.
(1) In view of the difficulty of finding an exact equivalent in English for the expression "de plein droit", it has been decided to translate it by the words "without notice," which represent the meaning attached to it by the draftsman as appears from the General Report see p. 44 ' (note in the original) '.
Art. 23. Articles exclusively used for war may be added to the list of absolute contraband by a declaration, which must be notified.
Such notification must be addressed to the Governments of other Powers, or to their representatives accredited to the Power making the declaration. A notification made after the outbreak of hostilities is addressed only to neutral Powers.
Art. 24. The following articles, susceptible of use in war as well as for purposes of peace, may, without notice (1), be treated as contraband of war, under the name of conditional contraband:
(2) Forage and grain, suitable for feeding animals.
(3) Clothing, fabrics for clothing, and boots and shoes, suitable for use in war.
(4) Gold and silver in coin or bullion; paper money.
(5) Vehicles of all kinds available for use in war, and their component parts.
(6) Vessels, craft, and boats of all kinds; floating docks, parts of docks and their component parts.
(7) Railway material, both fixed and rolling-stock, and material for telegraphs, wireless telegraphs, and telephones.
( Balloons and flying machines and their distinctive component parts, together with accessories and articles recognizable as intended for use in connection with balloons and flying machines.
(9) Fuel; lubricants.
(10)Powder and explosives not specially prepared for use in war.
(11)Barbed wire and implements for fixing and cutting the same.
(12)Horseshoes and shoeing materials.
(13)Harness and saddlery.
(14)Field glasses, telescopes, chronometers, and all kinds of nautical instruments.
(1) See note relative to Article 22.
Art. 25. Articles susceptive of use in war as well as for purposes of peace, other than those enumerated in Articles 22 and 24, may be added to the list of conditional contraband by a declaration, which must be notified in the manner provided for in the second paragraph of Article 23.
Art. 26. If a Power waives, so far as it is concerned, the right to treat as contraband of war an article comprised in any of the classes enumerated in Articles 22 and 24, such intention shall be announced by a declaration, which must be notified in the manner provided for in the second paragraph of Article 23.
Art. 27. Articles which are not susceptible of use in war may not be declared contraband of war.
Art. 28. The following may not be declared contraband
(1) Raw cotton, wool, silk, jute, flax, hemp, and other raw materials of the textile industries, and yarns of the same.
(2) Oil seeds and nuts; copra.
(3) Rubber, resins, gums, and lacs; hops.
(4) Raw hides and horns, bones, and ivory.
(5) Natural and artificial manures, including nitrates and phosphates for agricultural purposes.
(6) Metallic ores.
(7) Earths, clays, lime, chalk, stone, including marble, bricks, slates, and tiles.
( Chinaware and glass.
(9) Paper and paper-making materials.
(10)Soap, paint and colours, including articles exclusively used in their manufacture, and varnish.
(11)Bleaching powder, soda ash, caustic soda, salt cake, ammonia, sulphate of ammonia, and sulphate of copper.
(12)Agricultural, mining, textile, and printing machinery.
(13)Precious and semi-precious stones, pearls, mother-of-pearl, and coral.
(14)Clocks and watches, other than chronometers.
(15)Fashion and fancy goods.
(16)Feathers of all kinds, hairs, and bristles.
(17)Articles of household furniture and decoration; office furniture and requisites.
Art. 29. Likewise the following may not be treated as contraband of war:
(1) Articles serving exclusively to aid the sick and wounded. They can, however, in case of urgent military necessity and subject to the payment of compensation, be requisitioned, if their destination is that specified in Article 30.
(2) Articles intended for the use of the vessel in which they are found, as well as those intended for the use of her crew and passengers during the voyage.
International Humanitarian Law - London Declaration 1909 (external - login to view)
International Humanitarian Law - San Remo Manual 1994 (external - login to view) (section 103)
If Turkey could show that Israel had some part (including having a 3rd party do their bidding) in the bombing of their Naval Base and the death of several Sailors then that would be an act of war by itself and Turkey would have every right to supply armed escorts to any ship headed for Gaza as long as the cargo did not conflict with this list.
It would certainly make Israel show the complete list of what it considers 'banned items' rather than confiscate it all and then decide what they will keep and what they will let through.