A shark with a sword!
The sawfish is a very large fish, related to sharks and rays. Sharks and rays stem from a common evolutionary ancestry -- approximately 400 million years ago, there was an evolutionary split between bony and cartilaginous fishes, sharks and rays being examples of the latter. Sharks developed into the well-known nomadic predatory powerhouse we all know, whereas rays adapted a more specialized form well-suited for feeding on the rich food sources on the ocean floor. The first sawfishes came into being approximately 56 million years ago, and descend from an unidentified species of guitarfish (the guitarfish is sort of an intermediate form between shark and ray).
Sawfishes dwell near the ocean bottom, where they lie in wait for smaller fishes (like mullet). If a group of small fish comes nearby, a hungry sawfish launches itself at its prey and attacks it using the saw-like snout it is named after. The sawfish will slash prey with the sharp saw, and then return to the bottom to catch the injured or stunned fishes that sank down there after its assault. The saw also serves as a defensive weapon, which the sawfish will use to fend off attacking sharks (or fishermen).
The saw also serves two other purposes: The sawfish uses it like a rake to unearth buried prey (like crabs), and more importantly, it houses a sophisticated sensory system
which functions as a motion detector. Remarkably sensitive, the saw allows the sawfish to detect distant motion of incoming prey and predators, and even locate buried prey by sensing their heartbeat. While the sawfish also has decent eyesight, its prime sense is the motion detector -- vision is not that useful when you live in the darkness at the bottom of the ocean. A final note about the saw snout: The apparent "teeth" sticking out of the sides of the saw are not teeth at all, but dermal denticles -- specially adapted, sharp scales.
The sawfish is a very large ray, reaching lengths exceeding 20 feet. Unlike most fishes, female sawfishes are remarkable in that they bear live young. You can imagine that'd be quite painful with the saw and all, but in fact the newborn sawfish has a rubbery sheath around its saw -- and the saw itself is soft at birth, hardening shortly afterwards.
Unless you are a smaller fish, crustacean or someone attacking the sawfish, an encounter with a sawfish is not likely to be painful. They tend to be gentle, sluggish and docile animals, and prefer to lie quietly at the bottom the water and not be disturbed. Since their teeth are dome-shaped, they cannot tear prey apart, thus also restricting them to feed on creatures a lot smaller than themselves. Remarkably for a 20-foot predatory fish related to sharks, there is not even one recorded incident of a sawfish killing or seriously harming a human. If you were travelling around tropical areas and looking for sawfishes, you could find them in a number of different places: The sawfish have the unique ability to survive in both freshwater and saltwater environments, and they can travel from the sea into rivers and lakes at will.
Unfortunately, you'd probably have to look for a long time. The sawfish is a seriously endangered species, primarily because they easily get entangled and caught in fishing nets set for other species, and decades of accidental capture has severely depleted their numbers. Also, they are quite sensitive to pollution or deliberate modification to the shallows they inhabit.
Wow, you learn a lot of obscure stuff by reading into your former favourite free software window manager, don't you?