All the spa's pools are very hot. But I still enjoyed my stay.
Explanation:
Subject-Verb Agreement: Both subject and verb should agree with the number which means that both subject and verb should be either singular or plural.
The subject-verb disagreement: All of the spa’s pools refers to multiple pools, therefore, the plural verb should be used.
In the question, “spa pools” is the subject mentioned which should make use of the plural verb “are”.