Where can i buy a shark detector?

my dad just brought a 100 foot yacht...we went on a family vacation to Bahamas on the yacht we stopped in the middle of the ocean to go for a swim but everyone was afraid to go swimming an jet skiing because they where afraid sharks might be in the water....Where Can I buy A shark Detector That is capable of detecting sharks up to 10 miles away...or does a shark Detector Even Exist?