A swim can make a busy mind attend to one necessary breath at a time.

Certain activities do not require concentration, they impose it. In water, breathing has to be timed, and that single demand crowds out the running commentary no amount of intention could quiet. It is not that swimming clears the mind. It gives the mind something so immediate that everything else has to wait, which for a while amounts to the same relief.

If your thinking has been loud, choose something physical with a real requirement in it rather than something that leaves you free to think. The constraint is doing the work, so pick one you cannot ignore.