This is a little off topic as it is not an app, but in studying which AIS transponder to buy to replace my existing AIS receiver, I just picked the brand new Digital Yacht AIT6000 which comes with an Anchor alarm which is driven by the accuracy checked topside mouted GPS used for the AIS and wired into a physical alarm. I have had anchors drag(before I got more chain), so I do like the idea of an alarm which is more reliable than a phone app. The AIT6000 can also warn of an impending collision based on AIS traffic. Nice safety features which I have not gotten to use yet.