Maritime law is field of law which governs nautical issues. Maritime law is often split up into two categories: wet and dry. Wet law focuses more on legal issues regarding ship operations. This covers maritime accidents, salvage, and other issues that develop at sea, etc. Dry law examines what happens on land but still related to ships. This can include shore labour, the contracts of carriage, the loading/discharging of cargo etc.
If you steal a ship, you a committing a maritime crime. There are maritime and criminal elements to it. The prosecution for the theft would be criminal. However, the issues regarding any possible damage to the ship, insurance, loss of business, etc. would be maritime law.
For things that happen in the air, you have aviation law. From a public perspective, maritime law might be more famous mainly because admiralty courts were in a way the prototype form of international law courts. In Admiralty courts, they would listen to parties from different countries under different law, but came up with solutions under a unified law. Unlike aviation law, maritime law has that richer history.