In most cases, laws exist at the state and federal levels. For example, both the state and federal governments have laws concerning murder. But there are some areas of law that exist only at the federal level.
Maritime or admiralty laws are one example. These cover the rules over nautical matters. They concern everything from shipping rights to employee issues on ships or boats. The right to govern the waterways in the country all fall under federal maritime laws, which is a right derived from the U.S. Constitution.
Article III
The right for federal courts to handle maritime matters is in Article III, Section 2, Clause 1 of the Constitution. It directly addresses how cases of admiralty and maritime fall under federal jurisdiction. This clause overall speaks to federal jurisdiction in a variety of situations, but only maritime law gets a specific mention to ensure there are no questions about where the jurisdiction over these matters lies.
Reasoning
The reason why jurisdiction over maritime matters is so important is the Founding Fathers wanted to be sure to protect national interests. They also determined a need for standard rules and regulations, which is difficult to create if states held jurisdiction. Another goal was to protect commerce.
Additional points
Having the federal courts hold jurisdiction over all maritime matters also allows for standard court rules. Procedures are the same in all matters, and the rules do not change from case to case. Consistency was important to the Founding Fathers, which is why maritime laws are specifically mentioned in Article III.