NPT DM752-38 5" Non-Skid To Depth Marker helps to create a safe swimming experience for everyone. The depth markers are used to mark where it is safe to dive and how deep the water is. Enjoy these vibrant and bold depth markers for any swimming situation. helps to create a safe swimming experience for everyone. The depth markers are used to mark where it is safe to dive and how deep the water is. Enjoy these vibrant and bold depth markers for any swimming situation.