I'll write more later, but...
Could you, for now, stand her in the shower and use a jug/pitcher to pour water over her? (I see her getting cold as a possible problem, though.)
Ask for a hotel room with a tub since you're running out of time?
Does she play with dolls? Or do you have a dog? Could she give her dolls/dog a shower at first to get used to the spray? When it's less focused on her she may not freak out so much.
For the more general water piece: I swam competitively as a kid and take kids on swimming field trips a lot. Picture me with 25 kids, most of whom have never been swimming before and are also terrified of the water!
Sometimes the group effect can be helpful. Could you take her swimming with her cousins and explain beforehand that you are willing to hold on to her in the water the. entire. time.? Often they have so much fun splashing each other they forget to be afraid. When in the water, I hold them tightly so they feel safe and that helps. And they know they can always grab on to me at any time, any place they can reach (often around my neck!) when they get scared. I then move them around so they're on my hip or hanging off my shoulder instead!
In general, this is a time to cash in on some of that trust you've been storing up :) Tell her repeatedly, "I keep you safe. When you say stop, we stop. But I also know how much you like learning new things and how proud you will be."
Obviously it's best to find a pool she can stand in comfortably. The hotel pool might be good for this, but be ware of other kids who won't understand her fear. (hotel pools are smaller and its harder to keep your own space)
Teaching her how to float while in the water can be very helpful so they feel more in control in the water and less at the water's mercy. Begin by being in the pool with her (if she'll let you I guess) and then hold her upper body tightly and let her watch her legs float to the surface. Over time hold her less and less tightly until she can float on her back with your hand under the small of her back. This is one that has worked well for me. Typically encouraging them to make a "sea star" shape helps them float better than a "soldier/pencil" shape.
Do you have access to any babies? From your other posts I know she likes helping other kids, maybe you guys could take the baby swimming and she could help "teach" the baby how to swim and play in the water? Focusing on the baby may help her forget her own fears?
I taught my college roommate how to swim because she had always been afraid, and as a teen I taught the little kids to dive because they were afraid of our swim coach :)
More ideas: let her wear swim goggles in the shower. It helps stop the shampoo getting in her eyes too. My little sister loved swimming and taking showers but she always insisted on wearing goggles in the beginning.
She may like swimming more now than as infant since you can talk her through it now. Could she lay on her tummy in the bath and blow bubbles? Could she wear a snorkel in the bath just to practice putting her face in? Then you could try it at the pool? The bath is nice because there are no other kids around.
I'm only pushing the pool stuff because I think being able to swim is a life skill. I don't think she needs to love it or do it regularly, but a basic competency in the water could be, literally , a lifesaver some day. :/