Re: customizing maps for lowrance GPS
Lowrance MapCreate software could be used to add elements to maps, but I'm not sure about its availability any more, and it may be more trouble than it is worth. Alternatively, you can use a few waypoints around a mangrove (connected as a route) to outline the area, or just mark the general area using special waypoint icons (which mean mangrove islands to you).
Good Luck,