There are many PADI certified diving centers in Seychelles, with plentiful opportunities to dive on Mahe, Praslin, even one on Silhouette and La Digue. I would not say that it is the cheapest place to go diving, as the prices are as follows:  1 time 69 EUR, 6 times 310 EUR, equipment rent 12 EUR per day. However, the water is very warm and it is enough to have OWD certificate, as diving depth is only 15-18 m.