I use this App as well, and like me you seem to spend most of your time paddling at an elevation "above" sea level. Quite a good App though despite a few minor bugs, and at least Garmin has seen fit to include SUP as an activity in their options (only on their website , not currently on my device) so we no longer have to be kayakers like on some of the other similar offerings. The mapping is good as is the online dashboard. Wait till you do an upwind - downwind run and you'll see a huge difference in speed / distance with a big wind behind you. Enjoy - and try to stay "on" the water, not "above"
