Enlist lots of help.
Years ago I was involved in a video shoot to replicate a wedding reception party. We rented a hotel ballroom and we begged borrowed and (almost) stole to get things looking like a legit reception. We even had a bakery donate a wedding cake. We enlisted all our friends and had their friends come in with explicit instructions to come dressed as if attending a wedding. It was one of the biggest productions I have been directly involved and it scares me in retrospect how fortunate we were nothing critical went wrong. Did things go wrong? Sure, but we worked around it.
So, some suggestions; Enlist lots of help from friends and family members. They need to be clued in to the design process right from the get go so when it comes time to do their job you won't have to micro-manage them and they'll know what needs to be done. The hardest thing is to let them do their job. The size of your set will determine the size of your crew (grips, makeup, hair, riggers, etc.)
For location, I would first reach out through social media for anyone with a pool they are willing to let you use for free. Although you may be able to secure it for free, realize that there will be some expenses. The 'set' will need to be cleaned up and 'dressed' to look like a pool party. This means all the amenities as you mentioned. It's also nice to offer some money to the property owner, even if they're offering free. If you have multiple offers do some location scouting. Look for those tableaus where different shoots can take place. Pay attention to backgrounds but realize that some backgrounds can be manipulated with some set dressing or post production work.
Have people bring as many props as they can. They can also reach out to their social circles as well. The success comes from enlisting people who are just as engaged in the production as you are. If a person can find value in the photos you're taking then consider working out some form of trade. For example, hair and makeup. While the majority of the event will likely be about shooting for the clothing designer, you can take a few minutes to get closeups of the hair and makeup. Just remember that some images will go to the designer while some will go to the hairstylist and others to the makeup artist. Let them each have their own unique images and everyone will be happy. You may be able to work out a deal with a B&B for free location photos to use in their own advertising in exchange for use of their premises (on their off time). Create a shot list so you can keep track of all this.
If the client is not providing an AD consider enlisting a trusted friend to help you as a creative assistant. They can act as a second set of eyes for you.
The last bit of advice I can offer is, don't sweat the little stuff. There is always a way of working around the little stuff.
Have fun and we look forward to seeing the results of your shoot.