I am so sorry to hear about you and your Dad's experience.
I had a similiar experience on a 21 day panama canal trip on the Norwegian Bliss in Nov 2019.
When we left for the ship that morning, my Mom was normal, vibrant and alert.
Because of negligence on the part of the Bliss, after she arrived, but before I got to the ship,,,,and then into that evening after we sailed.
My Mom deteriorated physically and mentally.
Morning of Nov 3rd, she was great, the morning of Nov 4th, she was like a 2 year old child in EVERY way...and for the next 3 weeks, she did little improvement.
She got off the ship once, in Aruba, but that was bad cause she fell.
The entire 3 weeks were literal hell.
Bliss knew they screwed up and the had "babysitters" some when we were in port to stay with my Mom cause she could not be left alone for any amount of time.
It took a few months for her to get back to sort of being herself.... never fully did. But, that did not stop her from cruising...this upcoming 50 day Tales of the South Pacific will probably be her best, and her last.
It is sad when the cruising chapter of our parents lives come to an end. Happily, we can carry those memories in our hearts and photos, even when the mind fades over time.
I am glad you made so many wonderful memories with your Dear Dad 🙂