I am heading to the LM Khao Lak in May and really looking forward to the area. I Might be there too late for snorkeling (start of monsoon season), but I have also been on a day trip to Khao Sok, which is a national park in the mountains with some great hikes, boating on a lake, and other nature based activities. If you have enough time and looking for some land based activities -- I suggest going up there.

We just got back from a trip over thanksgiving where we stayed at the JW and LM Yacht club, which you are also thinking about. There are tons of posts on the JW and some about LM, but if you have other questions -- let me know. The Yacht club location would be great for day trips to Phi Phi for diving.