Diving is not as good as in the Red Sea and the caribs but the sea is worm and in some places viz is unlimited. usually though about 70-100 mtrs at 20 mtrs depth provided that the sea is not full of plancton. Wrecks quite a few in the North West of Evia
as well as in many other parts.
Sporades, as you can see from the attached chart, and also around Alonisos area is classified as a marine park.
At Nisos Piperi (SSE from Alonisos) boats are prohibited to come closer than to 2 miles from the shores as is consiodered the nesting ground of the Med Seal.
In addition, fishing is also strictly prohibited in all of the area and an armed coastgurad boat patrols the area 24/7. In this area you can find dolphins as well as the meddeteranean seal Monahous Monahous,turltles and other marine life. There are quite a lot of fish too. More than what you usually can find in greece ranging from gorupers and stingrays to yellow and congar eals, octopus, red mallet, catle fish etc. Beleive it or not there are pilot whales in Greece but South of Krete and south in the Ionian Sea.
Sporades IMHO is an interesting place to dive. Is a pitty that we forgot the housing for the video camera in the hotel otherwise would have take a great video.