Krunky
New
RSdiver:If you could find a day boat travelling South from Koh Lanta to Hin Daeng/Hin Muang then you could find some fantastic diving that rivals the Similans or Burma.
I've been there 4 or 5 times myself and you are almost guaranteed Mantas...singles or fairly large groups...enjoy! In this area, your best bet all around is to stay on Phi Phi. phi-phi scuba (www.phiphi-scuba.com) do a great job.
Another option not mentioned would be to stay at Krabi. The boats go to all of the same dive sites plus some other local spots. AoNang Beach is nice, as is Railey Beach...stunning actually.