In a swimming pool a costume seems better though not really sure why! Maybe more professional for a swim lesson?!
In the sun, bikini bottoms and a rash vest takes away the worry of burning. My girls wear long UV leggings and long sleeved vests but they are super pale!
But, if the sun not an issue…as long as it’s not 2 triangles which just seem silly on a girl with no breasts, and she really wants one then why not. The cropped top ones aren’t too ‘grown up’. I’m coming round to the thinking that if it’s practical and safe does it really matter! Too many people with too many opinions on girls clothing…myself included not so long ago!