Bonjour -
Was in Ko Lanta in November 2004. There is plenty of dive operators in the main town of Ben Saladin, the "capital" of the island. They all did 2 dive day trips to the dive spots off the Phi Phis. I joined one of them, it is like a 2 hour sail, and it was fun. Definitely not spectacular diving, but nice enough. Not sure whether the sites are still worthwile post tsunami.
The much better dive site in the area is Red Rock (i forgot the Thai name, Hin Deang or something - too lazy to look it up, es dimanche non). It is a rock in the middle of the Andaman Sea, four hour sail from Ben Saladin. We went there with a big boat with may be 30 divers, good fun. Good chance to spot manta's - the first group in when i was there indeed saw one.
If you want to stay on both islands I guess you can take the early morning ferry and do not lose any day diving (there was at the time a connecting ferry from phiphi to Phuket). We had a fantastic time in the southern part of Lanta, maybe 30 min ride from the main town. The Narima Bay, very friendly people. If you travel with a non-diving female companying, you can leave her behind at the Narima not feeling guilty, which is a piece of mind i like when enjoying myself u/w and knowing the lady is comfi as well. And the good thing is they have their own dive shop through which you can arrange everything - no stress for you, let them work it out.
Let me know if you have further questions, ciao.
Arie