The appointment confirmation advised to arrive one hour early to use the hydrotherapy gardens. I think this is what most people do. They walked us back to the locker rooms so we could lock up our stuff and put bathing suits/robes on, then DH and I met up in the hydrotherapy gardens which are co-ed. When we were done, we went back into our respective locker rooms to change out of our wet bathing suits and then we met back up in the co-ed relaxation room before our appointment time which is where the massage therapists picked us up. I think they want you relaxed and in the mood to enjoy your massage before they begin. Our massage was at 10am, so we got there around 9am and the spa was pretty quiet. The locker room and hydrotherapy gardens were busier when we were done with our service around 11am. So at that point, we just showered off and threw on clothes to leave. It actually worked out well, because by that time, we had the entire afternoon to enjoy the resort. However, you do have the option to use the gardens afterward if you want to.
I agree with you that 10am is the optimal time for a service. The other time I was debating was in the evening after dinner, around 6 or 7pm. I believe the spa is open until 8pm. Since there is a lack of evening entertainment and activities at the resort, I thought this would be good to do one night. But we went with the morning instead because I hate clock watching and having to be somewhere at a certain time. Also, I highly recommend taking advantage of the locker room facilities such as shower, body wash, lotion, etc. The locker room and showers are beautiful and the lotion/body washes smell amazing! DH loved his too!