Is it a waist of money, or a good idea?

That depends on numerous factors. Firstly, are you kayak fishing where there are a lot of sharks? if not, then yes, waste of time. If yes, then it still depends...

Are you nervous about sharks, and do you know what you would do if you had a scary encounter? If yes nervous, and no idea what you'd do, then yes, a SS might be a good idea. Even if the thing doesn't actually work (they do) at least you'll be more confident and less likely to panic yourself into further trouble.

I had a great white encounter once (actually, had several, but one well publicized) and I did start using a SS for a while. But later realised that I wasn't turning it on even when sharks were around, so I got rid of it. I do kayak in sharky waters, but am confident I won't fall out, so therefore I have little to worry about. But everyone is different.