That guy may have sustained cuts or scratches while crabbing. This kind of thing is very scary. I mostly confine my own aquatic activities to heavily chlorinated swimming pools. Although when I visited my parents at their condo in Florida in March, a day after a big storm, I was shocked to find a dead rat floating in the pool. We called the pool guy and he “shocked” the water which I think is just code for 2 or more gallons of Chlorine. Pool was closed for 24 hours.
When you swim in untreated fresh or saltwater, regardless of whether you have a cut or not, you assume the risk of numerous types of bacteria and other microorganisms. There is a certain amoeba that thrives in warmer, slow moving freshwater in the southern states that has caused fatalities. No open wound is needed, they swim upstream through your nostrils into the brain. And kill you. Only way to eliminate the risk is stay out of the water.