As long as you're looking at the natural critters like sharks and octopi, you might as well take a look at the anthropomorphic versions of them in Savage Species (as well as dire, giant or huge versions of them).
Not that you need dire or giant critters to give your players a challenge, of course. I was talking to a guy who worked at an aquarium who was discussing some footage of a giant (6') octopus that was eating sharks in their main tank...he also showed some footage of an octopus climbing from a quarrantine tank to another one about 20' away (across dry land) into another quarrantine tank to devour its inhabitants...then crawling back. He also noted that one of the aquarium's young interns had been attacked and nearly dragged into a giant octopus' tank- it grabbed her with 5 tentacles while anchoring itself with its other 3 and started pulling. It took several other people to extricate her from its grasp.
Smart, strong, and capable of squeezing itself into ridiculously small spaces- sounds dangerous enough for low-level PCs to me.
Another thing that has been hinted at in previous posts is the sheer number of dangerous natural but small critters in the Tropics. If you look around the world, you'll find things like crabs that swarm, some of the most dangerous jellyfish in the world (like sea wasps and Man O Wars), sea snakes, the most poisonous centipedes in the world, and one of my personal favorites, the poisonous cone snails.
Cone snails, having a number of species ranging from about 1-6" in length, have one of the nastiest neurotoxins in the world. They use it primarily for hunting fish with their harpoon-like radula (kind of a hybrid of tooth and tongue). Some have toxins so potent that they are quite capable of killing humans.