One of the main reason I use nautical miles and knots as measurement types while boating is convenience in navigation. As Strange Magic pointed out one degree of latitude is equal to 60 nautical miles. This means that one minute of latitude is equal to one nautical mile. When using a nautical chart distances can be taken directly by using the latitude markings on the sides of the chart. Just remember not to use the longitude markings at the top and bottom of the chart as one minute of longitude only equals one nautical mile at the equator.

Using nautical miles and knots makes it easier to communicate with other boaters as we're all speaking the same language.

I paddled a few times with a Swiss paddler that always like to use kilometers his reason being they always sound more impressive than either statute or nautical miles.