Reefs are full of specimens that can injure you through contact. Plus you lose body heat 17 times faster in water vs air. Some people should wear a 5mm suit instead of a 3mm suit.
Another problem with injuries is that the blood attracts all kinds of fish that bite at the wound. Blood also attracts the mighty shark. Mightily.
As for killing a fish by touching it.....balderdash. Ever been to Sea World? I made my living taking care of commercial aquariums.
I don't want to hijack the thread, but, there is evidence to suggest that handling fish can have a negative effect on them.
According to this report
from The University of Florida (emphasis added):
"Protective barriers against infection
1. Mucus (slime coat) is a physical barrier which
inhibits entry of disease organisms from the
environment into the fish.
It is also a chemical
barrier because it contains enzymes (lysozymes)
and antibodies (immunoglobulins) which can
kill invading organisms. Mucus also lubricates
the fish which aids movement through the water,
and it is also important for osmoregulation.
2. Scales and skin function as a physical barrier
which protects the fish against injury. When
these are damaged, a window is opened for
bacteria and other organisms to start an infection."
"Effect of stress on protective barriers
1. Any stress causes chemical changes in mucus
which decrease its effectiveness as a chemical
barrier against invading organisms. Stress upsets
the normal electrolyte (sodium, potassium, and
chloride) balance which results in excessive
uptake of water by fresh water fish and
dehydration in salt water fish. The need for
effective osmoregulatory support from mucus
components is increased.
2. Handling stress physically removes mucus from
This results in decreased chemical
protection, decreased osmoregulatory function
(at a time when it is most needed), decreased
lubrication thereby causing the fish to use more
energy to swim (at a time when its energy
reserves are already being used up
metabolically), and disruption of the physical
barrier against invading organisms."
So not 'balderdash' at all.
I would also expect that someone as intimately familiar with ichthyology as you profess to be would know better than to propagate the hysteria surrounding sharks.