What would be the best and most natural form for this sentence?
Miguel is wearing his favorite swim trunks.
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In my experience, the term 'bermudas' when used by itself has the same meaning in Spanish as in English: knee-length, light, walking shorts. A quick Google (ES) image search for "bermudas" seems to agree.
You might now and then see the term bermudas de baño, but usually bañador is preferred. Context is enough to know that you're referring to men's swim trunks, as that is by far the most common type of swimming apparel for men.
In Spain we use the term bermudas. The word bermuda or bermudas is used to refer to a kind of short trousers in general, both for swimming or not. If you want to specify just say bañador bermuda or add a context:
Miguel se está bañando con sus bermudas favoritas.
Well, in Mexico you would call swim trunks "traje de baño", which literally means swim suit. People also call it "shorts para nadar". And the most natural natural way of saying "Miguel is wearing his favorite swim trunks"in Mexico would be:
"Miguel trae su traje de baño favorito""or "Miguel trae puesto su traje de baño favorito"
If you say bañador instead of traje de baño in Mexico, people would or may think that you mean someone who showers or bathes other people.