Skiathos is reasonably close to the mainland and can be reached by ferry.
Ive dived there about 6 times in the last 3 years total about 20 dives depending on the weather you may get to dive a little island called Argo but other than that the law only allows the operator to dive Tsugriaki which is a mixture of sand an roack reef as with all site in teh med the fish life is not much
Octopus Diving Center on Skiathos – Skiathos Diving Center – Diving in Skiathos Greece –Free Diving in Skiathos island
There is no Nitrox or Tech catered for and dont try and keep up with Theo the owner he must have giles or a trade secret from being in the Greek navey as a Seal.