Conventional law / Customary law
The law of the sea is characterized by the cohabitation of two forms of expression: the law of the sea said to be "conventional" when it is contained in a treaty ratified by States which then become Parties to it, and customary law, both expressions being capable of cohabiting.
The United Nations Convention on the Law of the Sea (UNCLOS) is the conventional basis for the law of the sea in the Mediterranean and elsewhere. But independently of this law, rules inspired by practices pursued by the States, but not expressly emanating from the Convention and without the States being Parties to it, form the basis of customary law (see the case of the United States and the Exclusive Economic Zone).
Customary law thus relies on common rules whose origins do not lie in the Convention but rather in practices adopted by the States. The more these practices are generalized and repeated, over more or less time, the more the rule is considered as being accepted, without it necessarily being based on any particular article in the Convention.
The difficulty with customary law lies in the diverse ways in which it is expressed: unilateral acts passed by the States, resolutions adopted by international organizations, notices and decrees emanating from international jurisdictions. Everything then depends on circumstances - given that two factors are required when proof of the existence of a customary law needs to be established: it must be a practice accepted as being the law. Repeated over time, it must benefit from the most widespread application possible. Proving customary use is always a delicate undertaking. In the last resort, it falls to international judges to demonstrate its existence when disputes arise between States: some claiming the existence of the custom, others contesting its existence by formulating an objection when a State issues a unilateral decree. This objection needs to be persistent, and thus repeated on each occasion.
To give two examples: the notion of a continental shelf and its legal regime are indeed defined in the Convention , but a State which is not a Party to the Convention recognizes the continental shelf and may claim to exercise the sovereign rights implied by this notion, as this is quite customary. The second example is that of the Exclusive Economic Zone (EEZ)  which, however, is a concept officially created by the Convention of 1982. The International Court of Justice (ICJ)  has stated that this concept had customary existence through the effect of the multiplication of EEZ creations all over the world, well before the advent of the Convention.
This duality of a conventional law and a customary law makes action easier for practitioners, who no longer need to wonder whether this or that rule is applicable, as long as they are entitled to associate it with either one expression or the other of the law of the sea. Nevertheless, conventional law, ie. the written law, offers a safer guarantee of legibility, stability, and thus of application of the legal rule.