I asked this question at support@secondlife.com, and Pierre-Luc Linden suggested I post to the forums instead - here is my question:
"..asking about the problem of trying to sail a boat in SL for any distance without being bumped out by areas that have been banned by their owners. It's an infrastructure issue. Ideally, I (and other boaters) would like to see much more public Linden water, especially around the outer perimeter of the mainlands and between the islands, so that it would be possible to "sail around the world". Some residents are kindly putting up ATON (Aid to Navigation) markers
http://en.wikipedia.org/wiki/Navigational_aid
to help sailors maneuver around rocks and banned areas (the best and simplest is a red cylinder planted in the water), but that's rare, and we can't put them in Linden water. Some inland waters have a public Linden channel, many don't, or don't seem to - if I sail through a banned area trying to find the navigable water, I am bumped out, and have to log out to get control over my avatar again at best, or deal with a crash at worst. ... Is there any way of finding and charting navigable waters? Is any one working on this? Can I help?"
I've given it more thought since - one way might be to have another tag in Map to show parcels that have been banned for travel-through by their owners. And (longer term planning) more Linden sailing water around the mainlands and between the islands, where possible, that also shows up in the Map.
Resident participation could include recruiting sailors to put red cylinders at the key spots in the Linden channels (that would be fun!), or even SL Coast Guard to deal with inconsiderate sailors.