If you're prone to seasickness then the day boats from Koh Lanta might not be the best for you - I haven't been, but from what I found out it's at least 30-45 min to the dive sites. Of course it's up to you, but do look into the boat types - maybe some are better for you than others

As someone, who's always the first in the water and the last out, just because I normally do not get seasick in the water I do sympathise with the no LOB, but funnily enough I've rarely been seasick on a LOB, so maybe look into a (very short) LOB for your last part of the trip? They depart from Phuket or Khao Lak, and it's possible to do 2n/3days of diving.
We did not like the diving from Kata Beach, but only did 2 dives with Rumblefish, they gave us a very new guide, who was not able to find anything interesting - the things we did see, like 2 mating cuttlefish and some nice nudies, I found - on the other hand his under water navigation was flawless and he got us in/out of the water exactly where we needed to be.
Enjoy Thailand-