Around the same time as his writing of De iure belli ac pacis, Hugo Grotius wrote a short tract: Introduction to the jurisprudence of Holland. He was the first to offer a systematic account of the substantive law of Holland, with a specific focus on rights over laws. I propose that by drawing a comparison between the Introduction and his other major law treatises, the key elements and differences of his argument in Mare liberum become clearer. Secondly, it shows a different side of Grotius as a legal theorist, one concerned with smaller legal issues surrounding property relations in Holland, where potential conflicts stemming from water damage are particularly common and likely. By focusing on how Grotius handles categories of smaller bodies of water, in Introduction but also in ibp and De iure praedae, his understanding of what makes some water susceptible to ownership is drawn out.