An anchor alarm has one job: wake you when the boat is somewhere it shouldn't be, and never stay silent when it should have spoken. Everything in the design follows from that.
The watch circle
When you tap Set anchor in Sailing Log, your GPS position becomes the pin. You tell it your rode length and a safety margin (default 20%), and the alarm radius is rode times margin. Boat inside the circle: all quiet. Boat outside: alarm.
Filtering jitter without missing a drag
GPS positions wander a few meters even on a boat going nowhere. The alarm ignores fixes with poor reported accuracy and requires two consecutive fixes outside the circle before it fires, enough to reject noise, fast enough that a real drag alarms within seconds. The bias is deliberate: a false alarm costs you a grumble; a missed drag costs you the boat.
Being heard
When it triggers you get a looping siren that plays with the phone locked and ignores the silent switch, repeating notifications, and a full-screen red alert with the live distance. "Stop alarm" silences but stays armed, so a sleepy tap can't accidentally end the watch; "Lift anchor" is the deliberate act.
While you sleep
A Lock Screen Live Activity shows current distance against the radius all night. Half-awake check: glance at the nightstand, see 18 m of 36 m, go back to sleep.