A few things to point out to all you readers out there...
Similan day trips and overnight trips start in November from Phuket, so in September you have day trips around Phuket and Phi Phi. Some longer liveaboards start in October (3/4 day trips).
The day trip boats are large, comfortable (unless full, when you may feel a bit crowded) and not that slow actually, just they are not speedboats. Most dive shops (like the one I work for) don't have a boat. We book with those that do, so can get customers to different sites on the same day. Most trips have staff/students/divers who booked with many different companies and all paid the same price.
Look out for "OFF the stRIP" prices. (I think you mean, champ) You can book with little tour operators with shacks along the road. If so, then beware. They know nothing about diving except the price of the trip and how much discount they can give while still making a few baht commission. They cannot help you with equipment, they cannot answer any questions about dive sites, marine life etc.. Much better to book at a proper dive centre with staff who are pros. Maybe the little tour shacks offer you a cheaper price, but at what cost...
Visibility...variable on daytrips around Phuket. Average vis. for dive sites as follows ;
Racha Yai - 20 - 25m
Racha Noi - 30m
King Cruiser - 10- 15m (can be worse or can be better some days)
Shark Point/Anemone Reef - as King Cruiser
Phi Phi - 15m - as above.
Vis mainly varies with currents, heavy rainfall, and especially tides. Phi Phi and King Cruiser/Shark Point area can be less than 10m or can be 30m.
No one can guarantee weather or visibility!