What the arrow is telling you is that you passed the waypoint that you are navigating to. When in gps mode the arrow normally shows you the direction that the boat is heading. In navigation, it is showing you which direction you should be heading to get to destination. If the arrow is pointed toward the boat then you past your destination.
In the respects, of the alarm, there is an arrival alarm distance adjuster in the alarms menu. I believe it goes from 0-3000 ft. Hope this helps.