Freediving to the air bell at Blue grotto is certainly possible, but not real wise, as it's about 15 feet back under solid rock at the 30 foot depth level. (No direct access to the surface.) See the diagram attached below.
http://www.divebluegrotto.com/
I teach scuba classes there when the weather is bad, and I always wear long freediving fins. After making my class dives, I freedive for a while. It's 40 feet deep in the open basin area.
Devils Den is harder (creepy) to freedive in because of the limited light in the air cavern. It's really hard to see where the surface begins when you are ascending.
http://www.devilsden.com/
There is really no reason to mess around with compressed air ascents while freediving. Get scuba certified first, and after that if you still feel like you have to try it, (the novelty will likely be gone by then), do it from your tank, in very shallow water.
Attached are two photos looking up at Devil's Den, and one of my "freedive equipped" scuba students from Blue Grotto this year.
Chad
