You probably already know that most people who scuba dive go to Koh Tao and make that their base, as there are a lot of good dive sites off of Koh Tao. But can you stay on Koh Samui and dive from here? Absolutely! While the scene on Koh Samui is not as big as Koh Tao, some people choose to stay on Samui. The main advantage being a wider range of accommodation and eateries.
And not every good dive site is close to Koh Tao, in fact, Sail Rock which is arguably the best site in the area, is close to Koh Phangan and requires a half day trip from Koh Tao anyway. You can make a similar early morning departure from Koh Samui as you would from Koh Tao and dive there. There also sites around Angthong National Park and further south from Koh Samui that are much better accessed from Samui, making the options quite interesting.
All in all, yes, if you want to scuba dive it is totally possible from here and you have many options on where to go. There are quite a few dive centres on Koh Samui, so be sure to contact them and see what they offer and what their timetables are.