Did you ever get an answer about the showers? I know on the other ships you can, I am also curious as to this ship since we did not need them the first time but will this time. I did find this on their site
https://disneycruise.disney.go.com/faq/adults/fitness-facility-services/
I was on the Wish last month and was slightly disappointed with a spa shower situation.
To be clear I did not use the showers in the spa, and was told that they are available to use by anyone.
Here is the problem. The gym and spa while in close proximity are seperate areas on the Wish. The gym opens at 6am, but the Spa does not open until 8am, at least in March. I have been on all the other ships and just get a shower after an early morning workout, leaving my family to sleep in the room.
I was very surprised to not have access to a shower at 645 after a very sweaty workout. I was left with unappealing choices of going to breakfast in soaking wet clothes, or showering in my room with 3 sleeping family members.
I ended up grabbing breakfast and taking out to the abandoned pool area.
In short, I was told I could use the showers without any spa things booked, but not before 8am.
I think if the gym opens at 6am, the showers should be available by 630....
I did make my feelings known to guest services and on my feedback form.