Traveling the grid using a vehicle is awesome. It's one of the most pleasant casual activities to do with friends, has an incredibly low barrier to entry, and is often the first suggestion for people who don't know what to do in SL.
But traveling the grid via a vehicle is incredibly challenging. Security orbs, banlines, region crossing failures, the dreaded '4-way' corner of death.. it's all out there waiting for you. When traveling by car, bike, or other road vehicle it's usually pretty easy to stay on the roads. It's rare that a parcel boundary will claw out into the easily marked thoroughfare.
The same can not be said for boats.
Many areas of Second Life's waterways are just a patchwork of privately owned 'water' land.. with treacherously tiny little 'mole channels' winding through them. When it's a straight line, it's easy to follow, but when the path winds and curves, the edges are almost impossible to see until you've already crossed them. Third party viewers like Catznip and Firestorm have the option to show property lines on the minimap, but without a live-updating picture of your vehicle on the map, it's still only guesswork as to how close you actually are to the lines.
My suggestion is simply this.
If your camera is above the water line, the property lines (when enabled using CTRL+ALT+SHIFT+P) should render flat on the top of the water, once they reach the water line. When you are underwater, or if you have "render water" disabled (CTRL+ALT+SHIFT+7), the property lines should render on the land mesh below the water line instead.
See attached screenshots of the property lines as they currently render, and how difficult it is to navigate a treacherous thin path bordered by private property.