The verb form does not relate to the number of people you can see - it relates to the interrogative pronoun WHO, which is a singular noun. Whether there are two people or two hundred people swimming is irrelevant. 'Who' is singular, so the verb has to be singular to agree with it. Note that 'what' is also singular. 'Who is living in that house?' Mr and Mrs Brown 'What is in the basket?' Apples, pears and bananas.

i think you should ask ur teacher whenever u're confused but anyway, i think if you want to ask about people who are swimming, the question could be that who are swimmers? swimming is a noun, so i guess ur teacher uses it as a noun in this question "who is swimming". Furthermore, I've never seen any questions like "Who are swimming?"Even when i rewrite, it seems weird to me