Basically you’re talking about 2 things when you talk about the graph showing the boat position and the lake map. Showing the boat position is GPS. Showing the lake map is mapping.
There are some small inexpensive units that only have gps capabilities with no mapping. With these, it’ll basically show your boat’s location on a blank screen, but you can still mark waypoints and save your boat tracks for future reference. An example of a unit like this is: https://www.dickssportinggoods.com/p...strkr4xxxxxfel
With gps and mapping, you’ll see exactly what you’re asking for, which is your boats position in relation to the lake’s contour lines. Mapping is a bit confusing though because each graph manufacturer treats the maps differently. Some graphs only come with maps preloaded into them and some graphs accept an SD card (basically a memory chip) that has lake maps on them. I would suggest going to a reputable store that you have confidence in and get them to help you select the proper graph with the mapping that you need. Some of the preloaded maps don’t have certain lakes on them and some SD cards don’t have certain lakes on them either, so you need to know what lakes you plan to fish and whether or not what you’re buying has those lake maps.