I dived a 6.5mm Scubapro wetsuit, trip was in July. It was doable but putting a wetsuit on every afternoon wasn't nice but in the water fine. Boat had a drying room so was good for the next morning. Dived the usual suspects to ~40m, I wasn't doing long dives, max 1hr, never got the chivers. I wouldn't let this put you off a trip. Later is obviously better timing wise but then you start to run into weather issues topside (Oct>)