There are tons of operators, but the standard tours can be a real cattle drive in high season, with usual stops on Phi Phi Le and Phi Phi Don terribly crowded. If you're talking August then that's a monsoonal time and less crowds (though some) and weather and choppy seas can be a factor. The usual advice is to favor the east coast (Samui, etc.) for that time of year, though Samui is also massively more visited than it used to be.

I can't offer much in terms of positive advice (the operator I went with last time was cheap, but not what you're looking for). Just beware crowds.