Fenix 5x plus inaccurate tracking

So pretty much this morning I was at the beach and I went for a swim to see around the area for the first time. I selected the "Open Water Activity." After no more than about 8 mins I decided to see where I was with the map, but I bizzaro saw I was on the coast instead of the water. Its strange because yesterday when I went for a breif swim with my dad for 20mins this didnt happen. Btw I've had the fenix 5x plus for about 4 days now so maybe I did not activate something. Any ideas?