B’cvod HaRav,
Is a swimming pool considered בורות שיחין מערות ?
If a sheretz like a larvae grows in it, is it mutar?
Answer:
It is clear from the Rambam and other commentaries that the fact the bor is filled with water doesn’t make a difference.
However, because the water in swimming pools is not standing water, and there is rather a constant flow of water, it will apparently not have the status of a bor for this purpose.
For mekoros on the subject please see here (the paper relates to the question of copepods in NYC water, but many of the sources will also be relevant for the question asked).
Best wishes.
