This is the most common version of the Rhyme:

Jack and Jill went up the hill

To fetch a pail of water.

Jack fell down and broke his crown,

And Jill came tumbling after.

Up Jack got, and home did trot,

As fast as he could caper,

To old Dame Dob, who patched his nob

With vinegar and brown paper

There wasn't any well mentioned. Odd, huh? People assumed there's a well on top of the well because the two kids went "up the hill to fetch a pail of water" but nowhere was a water well explicitly mentioned. The kids simply went up the hill to fetch a pail of water sitting idly by.

Another point in case- watering wells were not innovated until the early 1800s. The original Rhyme was created during the late 1700s so there's no way the author had any idea of what a well is, let alone know enough to place it on top of a hill.