When I was diving on Koh Tao, some years ago, with Sairee Cottage Divers, they had two boats - one for students, which started at the usual early hour, and another for fun divers, which started at something like eight or nine in the morning, went out for three dives, and came back around five in the evening. I don't know whether or not they still do this.The one downside, so far found in Koh Tao, is all operators start too early; 09:00 would suit me.