Answer:
There are various reasons.
Step-by-step explanation:
very cold temperatures - the water can be colder than expected, even in warm weather
hidden currents - there can be strong underwater currents, which can trouble even the most confident of swimmers
reservoirs are often very deep - it is difficult to estimate depth and they can be deeper than people expect
there may be hidden debris or underwater hazards which can cause injury, including weeds and plants which can entangle people under the water
if it’s polluted it could make you ill
it can be very difficult to get out (especially steep slimy banks)
there are no lifeguards on duty
the remoteness of some of these places can also hamper and delay rescue attempts
I hope that helped!