Sorry – there ain’t no off-the-beach snorkelling on Ko Lanta.
You can book day trips (with snorkelling) to Ko Phi Phi, Ko Rok and the Trang Islands (typically Muk, Kradan, Cheuwak and Ngai).
It’s also a good start-point for day-dive trips to Ko Haa or Hin Daeng.
But no off-the-beach snorkelling.
I can’t say I’ve checked every square-inch of the island, but I’ve tried several spots and only found flat sandy bottom and low underwater visibility.