What a butt muffin.
I think you were fine.
That said, I think you should spell it out for him that you are not going to pay for your XH's activities (or DD's) unless XH works it out with you in advance. Furthermore, any financial transactions are between you and XH and you will not be giving money to a third party.
However, I do worry somewhat that the friends might take it out on your DD. It might be worthwhile to agree to pay for half of DD's admission this time (emphasizing that you will not in the future unless you've made prior arrangements). Bonus: if you look into it and realize that DD is young enough that she should have been admitted for free, you can act completely baffled that XH would have borrowed money for DD when he didn't need it.
"XH did not make prior arrangements for me to fund this excursion. He is not my dependent, so I'm not going to fund his activities unless we work something out ahead of time. In the future, I'd like him to discuss admission for our daughter ahead of time. I don't want you to resent paying for my DD, so I'll reimburse you for half of her admission this time. I'll call the water park and have a check in the mail as soon as I can. In the future, though, don't assume that I have agreed to pay for activities that don't involve me. If XH needs to borrow money, it's between you and him. It doesn't involve me."