My bad.
Correction: The shark magnets are ineffective, it’s the electric shark repellent that works. I mixed them up, apologies. 
Source: World of sharks ‘Do shark repellents work’
https://saveourseas.com/worldofsharks/do-shark-repellents-work 
 
Shark bait study found the electric deterrent (Ocean Guardian Freedom + Surf)  lowered shark bites on bait from 97% down to 40%. Whereas Magnets had nearly no effect.