Doing the Hot Tub Etiquette walkthrough will get this one.

Slight variation of Hot Tub Etiquette
Start with Pizza Dinner or Steak Dinner this time:

Go to the bedroom to put on a swimsuit
Wait for her in the hallway
Get in the hot tub
Suggest playing “Truth or Dare”
Truth  or  Dare (does not matter)
Yes  or  No or Finish dare (does not matter)
(Pick a question, does not matter)
(wait for her to finish answering)
Dare
End dare
Dare her to take her top off
Dare
Dare her to get naked
Kiss her nipples
Grab her butt
Rub her pussy

All you need is for Ariane to be dressed with at least 3 drinks, then ask to go swimming. So you can do Steak Dinner

Go to the backyard
Have a drink
Go swimming

Go swimming, challenge her to a race, look at the telescope and get in the hot tub on the same date.

Doing the Pool Hijinx walkthrough will get this one.

Do the variant of The Rebecca Trilogy 1 – Marco Polo where instead of swimming you go inside for drinks.

Do The Rebecca Trilogy 1 – Marco Polo but when its your turn:

follow the noise

and you will end up grabbing Rebecca’s breast.

This one is covered in Truth or Dare walkthrough

This one is covered in Pool Hijinx walkthrough. You must win a race.

Get Ariane to skinny dip, then challenge her to a race.

Purposely take too long clicking A B C so that Ariane will finish ahead of you. You get a nice view from behind.

See Pool Hijinx for more info on how to do this.

Start with Start of a Romantic Evening

Dance
Keep dancing
Protect Ariane
Drive back to Ariane's
Go to the backyard
Check out the telescope
(get the answer right)

Ariane will now be naked, achievement unlocked regardless of whatever else happens.

Go skinny dipping
Grab her boob
Just playing

For a more detailed walkthrough Thing To Do When You Are Naked