Ugh. I wouldn't try the day trip. it's a lot of travel for not much diving. I took one of those speedboats out to a big boat, but I spent the night on the big boat so I got 7 dives over 2 days instead on 2 dives in one dayI would also have a look at bigbluediving khao lak . They have a great boat Mv Hallelujah and a good crew of DM's.
They also have very fast boats (maybe the fastest) from Khao Lak to Richelou and Koh Bon if you want to do day trips
Here's the itinerary for one of the big boats that's served by a speedboat on the SImilans. The big boat stays out and the divers are transferred to and from Khao Lak via speedboat. It allows for very flexible scheduling, but you can see the issue with single day trips.
And here's the speedboat. Over an hour of this each way is not fun.