There is an old saying that if all else fails 'you can always sell it as a boat anchor'.

And then there are 'shark repellents'. Interesting marketing scheme. Make a claim. If they don't get eaten you claim it worked. If they do there isn't anyone left to claim the product didn't work.

http://physicsbuzz.physicscentral.com/2010/08/physics-of-shark-attacks.html

The search for shark repellents has been a long one. Just before WW2 someone figured that a mix of iron sulfate, presumably a bad tasting substance to a shark, and a dense blue dye, which would obscure the shark's vision or look like a dark wall, would repel sharks. Sounded good but nobody really tested it. The US Navy bought loads of the stuff. The primary effect was likely that it made people who jumped into the ocean feel a bit more confident that, just maybe, they wouldn't end up as a shark's lunch.

An inside joke was included in the packaging. You had a half-pound or so of the powder in a packet. On the outside of the packet, in large block letters were the words: "LIFE JACKET REPELLENT". Notice the complete lack of any mention of sharks. Taken literally its intended use was to keep life jackets away.

Some time after WW2 testing was done and it was found that sharks were not put off by the the taste of iron sulfate or the blue dye. No effort was made to check if it kept hostile life jackets away.

Then again these units have magnets and/or EM fields. And everyone knows you can do anything with magnets or EM fields because they are 'high-tech'. It has yet to be shown that the sharks are as impressed as the gullible humans who buy these sorts of things.