I think in terms of fear you actually have to look at the scenario of if you were swimming in their native habitat, the likelihood you'd be attacked, and the consequences.
If you're dead and decaying, yes, then you'd have to worry about piranha. However, if you're dead and decaying you don't have much to worry about. I think the only 3 freshwater fish I would fear would be:
1) Candiru catfish. For reasons stated above.
2) FW stingrays. You can very possibly step on one of these and it will to put it lightly, ruin your day (possibly life).
3) Electric eel. These are no electric catfish. However, I believe they're somewhat reclusive. Unless you go poking around for them, I think you're fine.
Your biggest worry in those types of environments are the crocodilians.
You want to talk real "monsters" I'm an offshore fisherman and licensed captain (of over 10 years). We routinely fish the gulfstream off the northeast USA. I've seem 12' tiger sharks that swim up to the boat and bite inanimate objects (floats, motors, gear, rudders, etc.) I've had 15'+ great whites cruise by. Witnessed schools of blue sharks so thick at night you can practically walk on them. Sharks by far and wide are the true aquatic "boogeyman"
If I had to pick a specific shark it would be the Tiger. They're clever and inquisitive. Not something you want a shark to be when you're in the water. After that it's bulls. We don't get them this far north though.