At least some swim meets use a light and a buzzer. I first encountered that many years ago at Gallaudet, a school for the deaf. I think that has become common at all swim races. It certainly makes sense to not exclude racers who can't hear.

I did note at the time that the gunshot sound put an extra oomph into the start over a buzzer. No idea whether that actually made a difference to my time, but it sure got my heart rate up.