Not for a shark alarm, but I remember being at the beach where my Dad was fishing in a gutter with a dozen other fishermen up to their waists in water and they had to get out because there was a shark (people chuck fish guts into the water when they clean their fish and it encourages the sharks).
Yes, but not so much an alarm.
I was visiting Green Island and saw reef sharks along the jetty to the Island so opted to stay out of the water, later I saw swimmers by the Jetty so swam over thinking it was safe.
When I got there the hair on my neck stood up on end as I looked down and beneath me were approx. 9 black tipped Reef Sharks.
I slowly swam for shore not making so much as a ripple in the water.
Needless to say I stayed out of the water for the remaining time of our stay there.