I need a GPS or Chartplotter to go on my boat and don't know which one I need. All I want it to do is to be able to mark schools of fish after I find them with my depth finder so I can stay on them. And I need to mark structure so I can return to it at a later date. I also want a unit that will mount on the boat insted of a handheld unit. I would like to keep price under $500. I have looked at several Garmin and Lowrance units and don't have any idea which unit I need to do the best job for the money. Thanks for any help you can give me.