A lien is a claim for an amount of money which is secured by a piece of personal or real property. A maritime lien entitles the lien-holder to have a vessel seized and sold to satisfy a debt. Since vessels can move, a maritime lien is enforceable by arresting the vessel and holding it in place until the debt is satisfied or secured with a bond. In the United States, this is always done by U.S. Marshals in an action in the federal courts. The law in this area is greatly complicated by the fact that maritime liens can arise without the lienholder filing the lien with any central authority. Only recently has the USCG accepted most liens for filing. Enforcement of maritime liens is also complicated by the fact that it courts to engage in the fiction of personifying the boat. Liens are enforceable against the boat itself, even though boats cannot be negligent, boats cannot breach contracts, and boats cannot fail to pay wages.

In the maritime sphere, liens arise in several areas. First, they arise when a maritime business provides "necessaries" to a vessel. "Necessaries" are those items or services that are necessary to the operation of the boat. For marinas, necessaries include dockage, fuel, electricity, water, hauling, etc. For service companies, liens will arise through repairs to the radar, engine, paint and the like. Liens may also arise from other areas, such as salvage, spare parts, and possibly even insurance premiums. In order to qualify, the parts or service must be necessary to operation, and they must be provided to the individual vessel. Generally speaking, items sold in a retail store, then taken to the vessel will not give rise to a maritime lien against the vessel.

Maritime liens can also arise in the context of injuries that occur aboard the vessel, from property damage caused by the vessel, and in claims for wages by seamen aboard the vessel. A passenger that is injured by the negligence of a crewmember has a lien against the boat. A dockowner whose property is damaged when the vessel comes into contact with it has a lien against the boat. A seaman, including virtually any person that is working on a vessel in navigation, will have a lien against the boat for his or her unpaid wages.

Enforcement of lien (for a marina or seaman) or defense of a lien (for the boatowner) can be complicated and costly. Suit must be filed where the boat is physically located, which can be difficult to ascertain. It may be advisable (or inadvisable) to assert a claim against the owner or operator of the vessel as well. Asserting a proper and enforceable claim to arrest a vessel requires significant expertise, and there are fees for having the marshals arrest the boat; fees for storing the boat until it can be sold; and attorneys' fees and costs associated with preparing and trying the case.

If you have a lien to enforce, or your vessel is seized to satisfy one, find an admiralty attorney, preferably one in the jurisdiction in which the boat lies. If you wait, you may lose your rights to enforce the claim, or lose you may lose your rights to contest an arrest that was done improperly.