Kyoto to Okayama and Naoshimi for day trip?

Hi,
It's really up to you and your travel pace. By garden, I presume you mean the Korakuen Garden which is one of Japan's traditional Top 3 gardens. Although the Handayama Garden is also close by and very beautiful. The former Mar-Sep is open at 7:30 AM.

In total it would take you about 2½ hours of travel to get to Uno for the ferry to Naoshima. So while quite hard, you could do it as a long day trip. But given that you are likely going to Naoshima with an interest of appreciating art, which is usually a more leisurely and time consuming activity, an overnight on Naoshima may be more satisfying for you. There is also the Benesse House which is part hotel and part art museum. Staying overnight offers one more possibility - the next morning, you could also go to Takamatsu, which has a number of great places, including one other garden which is also one of Japan's best - the Ritsurin Garden. The half day hike up to Konpirasan in Kotohira is also one of the finest things you'll ever experience.

So again, up to your interests, travel pace, and other plans.

Good luck.