Tonks is going to spend the next week in a bikini, most likely with a coconut in her hand. And she is so swimming with those dolphins.
Victoire will be wearing a modest swimsuit, still shy about the baby weight she hasn't completely lost yet, and a big floppy hat. She'll have Kitty underneath an umbrella with plenty of sunblock, and she'll sit on the sidelines of as many activities as she can find.
Snape will be brewing sunburn remedies and whatever other potions are needed. He will not be going swimming. Or be seen in a bathing suit. Or anything other than his standard set of robes. He despises the sun.
Merlin is going to be completely agog at the whole thing, and as soon as he sees those bikinis come out, his jaw is hitting the floor. But he'll happily go along with it, and once he discovers what modern bathing suits are like, expect him to be wandering around in board shorts, meeting as many people as he can.