Sure, hil71, you can get great daytrip diving out of Khao Lak by speedboat. Expect two dives per trip, and rather steep rates, though. Lanta is another good option, though the area around the Similans and Surins is quite a lot more extensive.
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Buadhai, a taxi from the airport to Khao Lak should not be more than 1200 Baht. At least I can arrange one for you at that price.