One that makes guys shriek is one you can't even see.
The toothpick fish. AKA don't pee in the water.
Normally it darts and heads for the gills of fish and spreads it's spiked fins to hold it in place in the gills. But it can swim up the urethra being drawn by the smell of ammonia. There it spreads it's fins and lives off the blood. Removal requires surgery, and possible amputation.
"I would rather be exposed to the inconveniences attending too much liberty than to those attending too small a degree of it." Thomas Jefferson