Hi,
I was in Tofo in August 2008. We saw Mantas several times at Manta Point and snorkelled with several whale sharks. We saw whale sharks on two days out of about 9 total days of diving, so there certainly is no guarantee of seeing them. August is very good for whales which you see from the surface (although two divers in our group were lucky to get a glimpse of them on scuba).
It was generally windy with rough seas, but we managed to dive every day. The water temp was about 21°C if I remember correctly and I was glad to have brought a semi-dry suit with hood and gloves. The boats did usually not go back to shore between dives because of the difficult launching so you had to stay on the boat between dives and you could easily get sea sick and cold, but then it is between dives that you go looking for the whale sharks.
It was a very good trip, but the conditions in August are rougher than the usual tropical diving.