A pool of standing water can be hidinga great many different kinds of bacteria. These bacteria are invisible to the human eye.
Pot hole
deep pot holes
Re-phrase the question - it is not clear.
Many hazards can be hiding in standing water. One of the most treacherous is sinkholes. Another danger in many states is flash floods. Even six inches of water can be hiding a current that will sweep your car away. (If your doing your your driving permit your answer should be "All of the above".)
Some time ago a child went missing not far from where I live . She was finaly found in a swimming pool full of stagnant water after nearl 2 days.
Water can stand in any low spot. As long as there is even a small area that is lower than the surroundings water can stand.
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Though the surface water is stagnant in a standing pool life may still thrive there. The beautiful Lotus flower loves stagnant water. The anaerobic bacteria is also found in stagnant water.
Pond, swimming pool, etc
Put a tarp roughly as big as your pool on it overnight if it is near a garden with good hiding places for small things.
Is the fountain Solid Brass or Chrome-Plated Brass?
It's all about how your body reacts to relative temperatures. The truth is that the pool temperature is not likely to be the culprit. It's more about how your body perceives the difference between the outside temperature vs the pool temperature. The transition from standing in the hot sun to getting into your pool makes the water feel colder than it might feel than if you are standing in cold rain before entering the pool.