The subject matter jurisdiction of admiralty courts is different from their jurisdiction in cases such as jurisdiction over the parties, the person, or the res involved. An Admiralty court may have jurisdiction, not only of a proceeding in rem, usually against a ship, but also of a proceeding in personam.
Admiralty courts have discretionary jurisdiction and pendent jurisdiction. The courts have discretion whether or not to exercise their jurisdiction in a particular case. Likewise, although an admiralty court cannot enforce an independent equitable claim, it may consider such a claim where it is necessary to do so for the complete adjustment of those rights as to which it has independent jurisdiction by applying equitable principles and remedies. However, the fact that only equitable remedies are sought does not deprive the Federal District Court of its admiralty jurisdiction.
Admiralty courts have jurisdiction, of all vessels or structures capable of navigating on water. Therefore, only vessels that are actively in navigation or temporarily withdrawn are subject to admiralty jurisdiction. Admiralty jurisdiction is not affected by the nationality of the res in a proceeding in rem in the absence of a statutory or treaty provision to the contrary.
Admiralty courts have jurisdiction over causes of action arising out of maritime contracts, either for direct enforcement or involving matters arising as the result of such contracts. Admiralty jurisdiction of a proceeding in personam on a maritime contract is not precluded by the fact that a proceeding in rem is not available. Jurisdiction is not dependent on the existence of a maritime lien where the claim is raised in personam. However, a proceeding in rem cannot be instituted in admiralty on the basis of a maritime contract not secured by a lien. Admiralty courts have jurisdiction over causes of action arising out of maritime torts. A maritime tort action is jurisdictionally established on diversity of citizenship.