Ko Lanta in July?

We are backpacking for three weeks in Thailand (July 2013) with our three children (8,5,5). Keen to spend some time at the beach. Would like to go to Ko lanta area but am wondering about the general effects of the monsoon at this time, in this area. It seems like a quieter alternative to Samui etc but we aren't keen to go to Ko Lanta if its too rough etc for swimming, snorkelling etc.

I have only been to Ko Lanta in january so can´t comment on how the weather is in july, but when we travelled in SE Asia we used www.travelfish.org a lot in our planning.

Pia