SweetSurrender--so much for the lovely welcome. It must be disheartening to join a group and find that one seemingly can't ask a question without having every aspect of her parenting or personality snarked about. Many seem to ignore the question completely and go straight for the "challenging statement," primed and ready for argument.
In my experience, it's perfectly normal and natural for 8-year-old boys to be aware of girls, and of women's bodies.
It is unusual, but not unheard of, for a swimming instructor to wear a bikini.
It is also unusual for a pupil to see his teacher in a bikini.
It is not unusual for children to ask questions of their parents when the children experience something new or unexpected (such as seeing a teacher in a bikini) and for the children to be able to describe what they have seen. (By 8, they should know enough "wow words" that would help the description along, yes, that's me being sarky now).
To answer your question, no, I don't think it was the most appropriate choice of swimwear for school swimming lessons. If nothing else, it's impractical.