There are always jellyfish in the waters of the lagoon... at least, every time we are there. They don't look as obvious as this one; I played with this photo to make the thing really show up. But there are plenty of them under the bridge and around the bulkhead. And they sting.
I've heard that vinegar helps the sting feel better, and the kids tried it. They said it helped some. Of course we didn't have plain white vinegar, we had apple cider vinegar. But I guess vinegar is vinegar, right?