I am sold on Anchor Pro. I have it set up on my phone and my iPad. If I am going to stay on the boat, I just walk to the bow with my phone and set my location when the anchor drops. Then set the distance before the alarm goes off.
If I am going to leave the boat with my phone, I do the same at the time I drop the anchor with my iPad. You have the ability to set up the anchor alarm so it will send an email if the distance is exceeded and you can see this on your boat. You also in the Premium setting can set up a way to remotely control the app (using Telegram...I haven't bothered).
Bottom line, download on both your iPhone and your iPad depending on whether you are going to leave the boat or not. I have more than once had the alarm blaring in my pocket as I pulled away in the dingy.
Hope that helps. Try the free version first. It is stupid simple to use.