Short answer: you cannot do this as a day trip. We do not sell one, and nor does anyone else worth booking. Koh Tao sits roughly 60 km north of the piers these boats leave from, and no Koh Samui day tour will come and collect you there.
The maths is the whole problem. The first ferry south does not land early enough for a tour check-in that happens at dawn, and the last ferry north leaves before the boats get back. Even if the timings looked survivable on paper, one delayed sailing and you have paid for a tour you watched leave without you.
Come down to Koh Samui the day before and sleep there. Next morning you use the tour's normal hotel pickup like everyone else, and the ferry cannot cost you anything. Plenty of people build a night on Samui into the trip for exactly this reason.
A private boat can be routed from Koh Tao, but it is a multi-day or bespoke arrangement rather than a day trip, and it is priced accordingly. Ask early if that is genuinely what you want.
Once you are staying on Samui, the tour comparison covers every boat we sell and what separates them.
No. Every tour on this site departs from Koh Samui and collects guests from Koh Samui hotels only. There is no Koh Tao pickup on any of them.
No. The group tours compared here use Koh Samui pickup zones, without exception.
Not safely. Tour check-in is early in the morning and the first ferry from Koh Tao arrives too late. Stay on Koh Samui the night before instead.