I live in the beautiful archipelago of Bocas del Toro on the Caribbean coast of Panama. I am on a small island, and tend to either swim or snorkel most days. And as yet, I have never felt threatened by anything "dangerous".
The video included here was filmed on and around the island, and includes much swimming and snorkeling. We've seen barracuda, octopus, rays, and even a couple of small nurse sharks. They have all been more scared of us than we are of them.
Every day in Bocas del Toro people swim, dive, snorkel, surf and play in the sea, and I've yet to hear of a serious incident. I imagine the most common sea creature-related problems are either cuts from getting close to the coral, spikes in a foot from standing on a spiny urchin, or an occasional sting from a jellyfish.
I live right by Dolphin Bay, and regularly see dolphins when out in my boat or canoe. But I've yet to meet them when I'm actually in the water. That would be fun.
Come on in, the water's lovely.