Blue Grotto is more of a cavern than a spring and it is a divers’ haven, so I did not sample it, but I wanted to see it anyway.  The cavern itself is stunning, even with the large business around it, which is an impressive operation with accommodations, dive sheds, a pool, a basketball court, etc.  I was happy that the staff generously let me just show up and take photos.  The limestone hole that makes the cave clearly has been modified somewhat, but it was not clear to me exactly how much.  In addition to the staircase leading down to the water, there was what appeared to be a retaining wall holding back soil on the bank.

The Blue Grotto cavern.  The retaining wall is in the upper left of the photo.

The Blue Grotto website posted this photo from 1962 (http://www.divebluegrotto.com/).  The water level looks much, much higher, but it may be that the photo was taken from the perspective of looking back toward where the retaining wall is now.  I have not yet found any data on its water level.