One of the most beautiful places you can visit is the isle of Capri in Italy. This has been on my must see list of places for as long as I can remember. Capri was once an ancient Roman Emperor’s escape. The blue grotto is a natural cave opening one can take a boat into (if the tide is low enough) and see some of the most crystal clear water around.
The glowing effect of the water inside the Blue Grotto is due to the sunlight which enters through a second, underwater cave mouth beneath that on the surface. For this reason, the most intense color comes with the most intense sunlight during the midday hours of noon to 2:00 p.m.. The difference between the first morning light and the late afternoon light is negligible, though there is often a longer line in the afternoon. It is best to avoid visiting the grotto on overcast days, when there internal glow is much less dramatic.
According to the Capri website: entrance to the Blue Grotto is possible only by boarding small rowboats which hold a maximum of four passengers at a time. The entrance ticket must be purchased at the floating ticket office at the cave entrance. After purchasing your ticket, your skipper will ask that you lay back along the bottom of the boat to enable the group to pass through the tiny mouth of the cave. Once inside, your skipper will row you around the cavern, singing traditional Neapolitan folk songs which echo off the rock walls and create an unforgettable atmosphere
No, visitors can neither enter the cave by swimming nor disembark from their rowboat to swim inside the cave. However, after the cave closes to the public at 5:30 p.m. (when the rowboats and ticket office close), many swimmers enter the cave by sea. This is both illegal and dangerous, as the cave mouth is low and narrow and in case of high waves it is easy to get thrown against the rocks.
More info can be found here.