Answer:
The water is the subject in the sentence.
Step-by-step explanation:
In any sentence, there is always a subject though sometimes the object is not included. The subject in any sentence is the one doing the action/ verb or who/ what the verb is based on. So, the subject in the sentence is the "water" which 'leaped'.
In the sentence,
The water leapt up from the center of the spring
the water is the subject, leapt is the verb which describes the action of the water and the spring is the object.