Hi Diane,
With regards to the best time for whale watching (and other animal encounters) on our South Africa tour this was covered in a recent thread st by Joy that you can find via this link (
viewtopic.php?f=3&t=839).
During the season our groups do regularly sea whales when we stop at Hermanus, though please be aware that this is not a boat excursion but viewing from the coast (however the whales do come surprisingly close to shore). I you really want to it is also possible to join whale watching excursions from Cape Town during your free time there.
I hope that helps,
Erik.