I have been using a Forerunner for several years to track my sailing for races and day trips.
Finally I decided to splurge and purchase a "Quatix" gps watch, which is specifically designed for sailing and boating and supports nautical units natively.
I was greatly disappointed when I uploaded my first track recording to Garmin Connect to share and discovered that Connect translated everything BACK INTO MPH!
If there is enough of a market to make a gps watch specifically for boating, I don't see why there isn't a market for an online tool to store and share the data collected from that watch.
Please add support for Nautical Units to Garmin Connect!