The waters in the back have them pretty much everywhere from time to time - Secret Beach (back when it was secret) used to be really bad - back then I would only get in the water if I had a floating device so that my feet would not touch bottom to stir them up. I'm guessing that frequent use/stirring up the sandy bottom has disrupted their habitat and made the area swimmable.
They do seem seasonal but not sure what the season is.
Glad they didn't ruin Phil's day and hopefully the numbers of these creatures remain low enough that everybody can enjoy this lovely spot. From my experience I can tell you that they DO bite, and in fact hang on and you have to pull them off. They love going inside your swim suit. I've wondered what Leonardo was thinking, buying Blackadore --- there is a reason old-timers call the area "sealice bay".