If I understood you well, you go up and down in a kind of grid with your boat and have a certain width to the left and right of your boat that you consider to be "covered area" - let's say a band with 5.5. meters width.
You can easily change the width of your line in either "map units" or "meters in scale". I defined a line-layer in EPSG 4326 (that's what you use for GPS, aren't you?) and set the project EPSG to 3857 (projected CRS with units=meters) or whatever is suitable for your area. If you have EPSG 4326 as project EPSG, than you can't use "map units" for line-width (since it's in degree). However, "meters" should still work.
Mark the line-layer, open layer styling, select the line and change the units from pixels to "map units" or meters - see screenshot. I defined a 5.5 meters width and selected a red, semi-transparent color: if the boat goes up and down in a 5-meter distance grid, you have overlapping portions (dark red stripes). Only for visualisation purpose, I added the thin black lines in 5 meter-distance to show the boat's route.