Not sure if anyone will benefit from this but i stumbled into it last week and it will work for any points in a map regardless of whether there are roads or not.
http://robaquatics.com/2011/02/how-to-measure-swimming-distance-in-open-water-with-google-maps.html
Here's one that is focused on running/biking but works on the water as well. It can show mile marks as you go.
http://www.gmap-pedometer.com/
I myself am a "mapmyrun.com" kind of guy
Wow! Good stuff guys... God Bless the Internet!! Where would we be without it!!??
Google Earth has a ruler in the tool bar that gives great accuracy and is easy to use
eragon wrote: Google Earth has a ruler in the tool bar that gives great accuracy and is easy to use
I measured my Hobie 18 in my Dad's backyard with it. Amazingly a Hobie 18 was 18 feet long.