The idea of marine insurance is the same as with other kinds of insurance in that there are two parties: the assured and assurer (or carrier). The marine policy may cover the risks of a single voyage, or may insure for a certain period of time. Goods or cargo are almost always insured by voyage. Boats are usually insured for certain period of time, usually they are contracted year by year. Cargo policies may be on a may be open to cover cargo as shipped by the insured. The insurance of boats, equipment and belonging, may cover a ship or a whole fleet. The marine insurance not only provides coverage of losses it also provides warranties for ships and cargos, “A warranty, as in fire insurance must be strictly performed. In marine insurance there are three implied warranties which are understood in every contract. They are in respect to seaworthiness, deviation, and legality. Seaworthiness is the condition of a ship when reasonably fit to perform the services and encounter the ordinary perils incident to the voyage. The second implied warranty is that there shall be no voluntary deviation or departure from the course fixed by mercantile usage, for the voyage contemplated by the policy; and also that there shall be no unreasonable delay in commencing or making the voyage. The third implied warranty is that the voyage shall be legal, both in its nature and in the manner in which it is prosecuted. Smuggling voyages and trading trips to an enemy's port are cases of illegal voyage.” In general, marine insurance policies are “agreed value” policies which mean that the insured and the insurance companies have already set a agreement of value for the ships and cargos. But in boat policy, such as yacht or pleasure boating are much like insurance policies in motor vehicles, does not set an agreed value and in the event of loss depreciation is usually deducted from the amount the insured will recover.

Reference: http://chestofbooks.com/business/reference/The-Business-Man-Encyclopedia/Marine-Insurance.html