They'd have to be pretty dim pirates not to recognise it as an electric fence and a few bits of wire is all they'd need to overcome it. Might keep sea-cows and sea-horses away, I suppose?

In some jurisdictions you might end up in deep poo if anyone was harmed as a consequence of the kit - e.g. 'pirate' falls into the water and drowns/harms himself. In the UK you would certainly be at risk of that (not that you'd want to use it in the UK, maybe, but could be that other countries have similar laws?