Israel has declared a blockade on Gaza and justified previous fatal attacks on neutral civilian vessels on the High Seas in terms of enforcing that embargo, under the legal cover given by the San Remo Manual of International Law Applicable to Armed Conflicts at Sea.

There are however fundamental flaws in this line of argument.  It falls completely on one fact alone. San Remo only applies to blockade in times of armed conflict. Israel is not currently engaged in an armed conflict, and presumably does not wish to be.  San Remo does not confer any right to impose a permanent blockade outwith times of armed conflict, and in fact specifically excludes as illegal a general blockade on an entire population.

It should not be denied that Israel suffers from sporadic terrorist attacks emanating from Gaza.

However this does not come close to reaching the bar of armed conflict that would trigger the right to impose a limited naval blockade in terms of San Remo. To make a comparison, in the 1970's and 1980's the United Kingdom suffered continued terrorist attack from the Irish Republican Army, with much more murderous impact causing many more deaths than anything Israel has suffered in recent years from Gaza.  However nobody would seek to argue that the UK would have had the right to mount a general naval blockade of the Republic of Ireland in the 1970's and 1980's, even though the Republic was undoubtedly the base for much IRA supply and operations.  Justifications of Israeli naval action against neutral civilian ships by San Remo is based on special pleading and an impossibly strained definition of the term armed conflict.